There needs to be a definitive thread for this issue in the TJ Resources section of this site. This issue is so common it seems, that I'd like a thread to be able to direct people towards so they don't have to search endlessly.
The Issue
After re-gearing some Jeep Wrangler TJs, you will find that there will be a "vibration" that wasn't there before. I'd like to start by pointing out that this vibration will be very different than the vibrations you'd feel from an improperly balance wheel / tire, or the vibration you'd feel from an incorrect pinion angle. This is what I would describe as a cyclical or harmonic vibration, that if measured on on Oscilloscope would have a very clear up and down pattern, similar to a heartbeat.
This vibration generally doesn't come on until higher speeds, though some members (such as @psrivats) have reported it at much lower speeds, which I can attest too personally (I rode in his Jeep, and it was terrible, even at low speeds).
The best way to test for this issue is to get on the highway and get going about 65 MPH. Turn the radio off and roll the windows up (make sure you have the top on as well). If the issue exists, you should be able to hear (and feel) a very distinct vibration that moves like a pulse, in-and-out.
Why does it happen?
I can't answer that. If someone has some actual science as to why this happens after a re-gear, I'd love to hear it. It's possible that the issue was there before the re-gear, but wasn't happening until much higher speeds (i.e. 90+ MPH), in which case no one would usually know the issue existed to begin with. Perhaps after the re-gear it just moves this vibration to a much lower speed.
Another thing that has crossed my mind is that maybe there are issues with the gears themselves, though others seem to have ruled this out (@bobthetj03 installed multiple sets of gears I believe, and they all had the same issue).
An interesting thing is that when you remove the front driveshaft on the TJs that do experience this issue, it seems to cure it. There are cases like @psrivats where removing the front driveshaft didn't make the vibration go away, but more often than not, it seems that removing the front driveshaft fixes the issue.
Either way, I'm still very curious as to why this happens.
Does it happen to all TJs?
No. I had both of my 2005 TJs re-geared (both were 42RLE transmissions as well). The first one was re-geared to 4.88 gears on 33" tires and had no issues after the re-gear whatsoever. I could take it up to 80 MPH easily without any vibration at all.
My second TJ I re-geared to 5.38 on 35" tires, and at around 60+ MPH, the vibration was so bad it was almost unbearable.
So the bottom line here is it's just luck of the draw. Some experience the vibration, others do not.
What's the fix?
The only sure fire way to fix this that I know of is to do what @toximus, @Mr. Bills, and many others have done, and install a manual locking hub conversion for the front. By unlocking the front hubs and taking the front driveshaft out of play entirely, this seems to completely cure the issue. The only downside to this is that if you do need to engage 4WD, you're going to have to get out of the vehicle and manually lock the front hubs.
I personally don't see that as being a big deal though, so this is what I would recommend to anyone who is experiencing this issue.
Another potential fix is to do what @psrivats did and take the vehicle to a shop that specializes in nothing but drivelines. Six States is the place that Sri took his, and they were able to dial out almost all of the vibrations. Actually, Sri thought they had gotten rid of them completely, but he reported a month or so later that they weren't in fact gone entirely, but had been greatly reduced.
He has been meaning to take it back to them as there is a very good chance they could dial out the rest of the vibrations, but he hasn't had the time
Resource Threads
Here's some different threads on this subject, all of which are loaded with great information:
https://wranglertjforum.com/threads...ations-after-re-gear-to-4-88-or-deeper.12218/
https://wranglertjforum.com/threads/harmonic-vibrations-in-jeeps-a-new-theory-please-read.15144/
https://wranglertjforum.com/threads/re-geared-now-i-have-a-vibration-above-50-mph.4235/
https://wranglertjforum.com/threads...ions-at-higher-speeds-after-re-gearing.32425/
Have anything to add to this thread? Please let me know. If it's good info, I'll add it to the original post.
The Issue
After re-gearing some Jeep Wrangler TJs, you will find that there will be a "vibration" that wasn't there before. I'd like to start by pointing out that this vibration will be very different than the vibrations you'd feel from an improperly balance wheel / tire, or the vibration you'd feel from an incorrect pinion angle. This is what I would describe as a cyclical or harmonic vibration, that if measured on on Oscilloscope would have a very clear up and down pattern, similar to a heartbeat.
This vibration generally doesn't come on until higher speeds, though some members (such as @psrivats) have reported it at much lower speeds, which I can attest too personally (I rode in his Jeep, and it was terrible, even at low speeds).
The best way to test for this issue is to get on the highway and get going about 65 MPH. Turn the radio off and roll the windows up (make sure you have the top on as well). If the issue exists, you should be able to hear (and feel) a very distinct vibration that moves like a pulse, in-and-out.
Why does it happen?
I can't answer that. If someone has some actual science as to why this happens after a re-gear, I'd love to hear it. It's possible that the issue was there before the re-gear, but wasn't happening until much higher speeds (i.e. 90+ MPH), in which case no one would usually know the issue existed to begin with. Perhaps after the re-gear it just moves this vibration to a much lower speed.
Another thing that has crossed my mind is that maybe there are issues with the gears themselves, though others seem to have ruled this out (@bobthetj03 installed multiple sets of gears I believe, and they all had the same issue).
An interesting thing is that when you remove the front driveshaft on the TJs that do experience this issue, it seems to cure it. There are cases like @psrivats where removing the front driveshaft didn't make the vibration go away, but more often than not, it seems that removing the front driveshaft fixes the issue.
Either way, I'm still very curious as to why this happens.
Does it happen to all TJs?
No. I had both of my 2005 TJs re-geared (both were 42RLE transmissions as well). The first one was re-geared to 4.88 gears on 33" tires and had no issues after the re-gear whatsoever. I could take it up to 80 MPH easily without any vibration at all.
My second TJ I re-geared to 5.38 on 35" tires, and at around 60+ MPH, the vibration was so bad it was almost unbearable.
So the bottom line here is it's just luck of the draw. Some experience the vibration, others do not.
What's the fix?
The only sure fire way to fix this that I know of is to do what @toximus, @Mr. Bills, and many others have done, and install a manual locking hub conversion for the front. By unlocking the front hubs and taking the front driveshaft out of play entirely, this seems to completely cure the issue. The only downside to this is that if you do need to engage 4WD, you're going to have to get out of the vehicle and manually lock the front hubs.
I personally don't see that as being a big deal though, so this is what I would recommend to anyone who is experiencing this issue.
Another potential fix is to do what @psrivats did and take the vehicle to a shop that specializes in nothing but drivelines. Six States is the place that Sri took his, and they were able to dial out almost all of the vibrations. Actually, Sri thought they had gotten rid of them completely, but he reported a month or so later that they weren't in fact gone entirely, but had been greatly reduced.
He has been meaning to take it back to them as there is a very good chance they could dial out the rest of the vibrations, but he hasn't had the time
Resource Threads
Here's some different threads on this subject, all of which are loaded with great information:
https://wranglertjforum.com/threads...ations-after-re-gear-to-4-88-or-deeper.12218/
https://wranglertjforum.com/threads/harmonic-vibrations-in-jeeps-a-new-theory-please-read.15144/
https://wranglertjforum.com/threads/re-geared-now-i-have-a-vibration-above-50-mph.4235/
https://wranglertjforum.com/threads...ions-at-higher-speeds-after-re-gearing.32425/
Have anything to add to this thread? Please let me know. If it's good info, I'll add it to the original post.