Driveline vibration only present when both driveshafts are connected

[BamaJeep]

So I posted a while back about driveline vibrations I am experiencing. To set the stage, it’s a 2002 4.0 with an unknown brand 2” lift. Had it when I bought it. Was having vibrations above 60mph. I replaced the rear u-joints, same vibration. New rear driveshaft, same vibration. I removed the rear driveshaft and the vibration went away completely. I don’t have adjustable control arms or SYE. Per the advice of someone here I tried adjusting the TC drop. It had a 1-1/2” drop and I tried everything between completely eliminating the drop all the way to 1”. Same vibration. So today I again removed the rear driveshaft just to see if I was crazy the first time and the vibration is gone. Put the rear back in and, just for kicks, removed the front driveshaft. Vibrations gone.

So what’s going on where the vibration is only present when both driveshafts are connected?

 

[chino1969]

This is a WAG but two possibilities are evident to me. 1) There is no problem with one driveshaft only. The cumulative effect of both drive shafts coupled together are enough to cause the vibrations. 2) There is a problem in the transfer case. I don't claim to be an expert and more experienced people will be able to offer more accurate assessments as well as offer additional troubleshooting or repair recommendations.

 

[K0LCB]

I would look for worn motor or transmission mounts. I had a similar problem, found a broke engine mount

 

[Gollywomper]

When chasing vibrations it is important to remove each DS and drive the rig. Just removing the rear and driving with the front still connected?powering the jeep changes the load on the front DS often masking the vibrations felt from the front DS when spinning under no load. So when we remove the rear and drive using front wheels the vibes go away, sometimes not because the issue was the rear, but because we changed the load on the front. If it drive good with front removed, you need to check front pinion angle and u joints for wear or stiff joints. If that doesnt take care of it I would suggest having the front DS rebalanced at a DS shop.
I spent a fair amount of time chasing down vibrations with similar results as you when removing either DS.

 

[BamaJeep]

When chasing vibrations it is important to remove each DS and drive the rig. Just removing the rear and driving with the front still connected?powering the jeep changes the load on the front DS often masking the vibrations felt from the front DS when spinning under no load. So when we remove the rear and drive using front wheels the vibes go away, sometimes not because the issue was the rear, but because we changed the load on the front. If it drive good with front removed, you need to check front pinion angle and u joints for wear or stiff joints. If that doesnt take care of it I would suggest having the front DS rebalanced at a DS shop.
I spent a fair amount of time chasing down vibrations with similar results as you when removing either DS.

Man that is such a great explanation of the potential problem and puts it in a perspective I hadn’t thought of. I’m going to take the front shaft to get it balanced and new u-joints and then go from there. Thank you for your help.

Edit: One of the u-joints on the front shaft is bad. It’s almost locked up. Other 2 seem okay but I’m going to replace all 3.