That video is interesting. Based on it, is it safe to say in your case, it is purely in the rear and has nothing to do with the front? I was going to throw out the totally out of my ass theory of two different length drive shafts working against one another. But after seeing the video, doubtful.
Unclear to me whether it's an issue only in the rear. I was told that when they initially saw the problem, they got the front shaft balanced and the vibes got slightly better (video is after they got the front drive shaft rebalanced and rebuilt). Now with the front or off, I'm told the vibes are not much different but I haven't seen it myself.
I think I look at this problem very differently than the shop and I'm not very sure how to interpret "not much different" or "slightly better". Being an engineer, I tend to think in terms of intensity (amplitude) and the duration (time period or frequency). Things either make a difference or not ... you may not be able to tell yourself, but there are instruments and appliances to use to help you tell that with confidence.
The standalone skid they tried (it was a cut-up stock skid) made things worse (vibes start coming at lower speed), and that tells me that the stiffness and weight of the skid do matter as
@Ranger_b0b pointed out. It's not known if the problem can be solved only with a heavier/stiffer skid. That's yet to be systematically tried.
The rear driveshaft is now getting checked. Note the it is quite new (less than 1k miles) and I had no vibes of any sort prior to regear. I've also been told both front and rear pinon angles have been thoroughly checked and are in good order. Note that these were actually set and checked by Dave himself earlier in the summer when I got other upgrades done.
The way the entire thing is having steady pulsed (i.e harmonic) vibrations tells me that this is the entire frame/skid/whatever is resonating at fundamental frequency and is what made me look into the physics more carefully.
I still don't fully understand what's running out of spec (so to speak) that's making the entire structure resonate. Jim Frens has hinted at some design choices but details are not 100% available for us to go make better choices.
Is it the torsional deflections in the driveshafts at increased rpms near half critical speeds that's the source of the problem? If yes, then I need better driveshafts that doesn't distort (stiffer one, either thicker tube or made of Al).
Or is it that we've somehow changed the system response to have these vibes in the skid/frame to be onset sooner, even though driveshafts and pinion angles are good? So do we need other mods to see (heavier/stiffer skid or in the weight fr damper) to stop the system from responding as it is doing now? There are plenty of reports of people just installing an SYE and the get what seem to be similar vibes that don't go away no matter how much they look at pinion angles. Makes me think the damper does play an important role. But why did the factory put them only in the rear?
Amidst all this, why do quite a few people report everything going back to normal with a front hub conversion? Quite a few also have reported the absence of the issue when they remove one driveshaft (front or rear, doesn't matter). What explains that?
So yeah. I have gaps in my understanding and I'm continually thinking and reading about this. I just don't have anything more concrete to add at this point of time.
I had actually emailed the shop saying that I was going to pick up my Jeep day after Thanksgiving. Dave saw that called me today in the morning and he told me that he has had other distractions and that he'll try his hand again at this problem. That's when I learned the front driveshaft is out for checking. There have been multiple un-communicated long delays from the shop side in attending to my Jeep, some of which have been very frustrating. We really need systematic controlled experiments to understand what changes we make elicit what response, but I'm having trouble making the shop understand the importance of being thorough and systematic.
I've been very patient (and
@Chris knows this) but I can only take so much after 6+ weeks of waiting with sporadic communication and then learning that they have not really worked on it or even tried anything that I've requested. Even learning what did not work is good progress in a problem like this. I'm still not quite sure what they plan to do and I'm going to be talking to Dave and Chris (the other Chris) again tomorrow.
One way or the other, I'll find a fix that works and more importantly understand why it works, either at that shop or elsewhere. I will say that I'm still very inclined to pull my Jeep from there and go elsewhere.