I was tagged on this thread so here I am. I only read the first couple posts though so if I'm saying things that have already been said I apologize.
I've put together a guide to troubleshooting drive shaft vibrations, with an accompanying video, here
https://4xshaft.com/blogs/faq/diagnosing-drive-shaft-vibrations?_pos=1&_sid=d6ec76a03&_ss=r.
It sounds to me like you have a dynamic vibration, something that is out of balance or rattling around a bit. Because you said the vibration was gone with the front shaft removed it signals to me that you either have a new shaft that could use a little improvement in the balance or that something the shaft attaches too is loose or wonky. Imagine that you have a loose output bearing on your transfer case, or that the nut is loose and the yoke is loose as a result. That yoke might be wobbling or rattling around (if it were spinning) even with the shaft removed but the yoke is only about a pound in weight. Not much potential for vibration. But when you bolt a 22 pound drive shaft to that wobbly yoke, now you have the much heavier drive shaft wobbling/rattling which has a much higher potential to cause vibrations.
I also think you have an example of harmonic vibrations. I think this because you said the vibration happens at a certain speed then goes away at higher speeds. This sounds like an example of multiple things that are vibrating all syncing up at a certain point, only to de-synchronize later. Every single part of your Jeep that is moving is vibrating after all, it is just a matter of intensity and frequency. A little more on that here
https://4xshaft.com/blogs/general-tech-info-articles/vibrations-harmonic. Now if you have 3 things contributing to the vibration I'm not saying you have to fix all 3 of those things, usually fixing the 1 thing that is the biggest contributor will have the desired affect. I'm just mentioning this as a reminder that it can be beneficial to remember to pull back a little and look at a broader range of things sometimes. That and because even though replacing 1 thing sometimes doesn't fully eliminate the vibration it doesn't always mean that that 1 thing didn't need to be replaced, just that it wasn't the only thing that needs to be replaced.
You can also call us for more guidance if you'd like. I saw that you got the shafts from Adams, I'm sure they'd be willing and able to help you as well.