Driveline Vibration in 4WD - Sierra 2500

if all your u-joints are good then i'd have to suspect a transfer case problem like a bearing/bushing. they could try to push up and down on driveshaft to see if there is any play in it. also check the u-joints in the rear as it doesn't seem to go away when in 2wd either. also check shafts going to front wheels and all wheel bearings.

The rear u joints are new too. This definately is directly related to the 4wd. It only happens when I engage the 4wd. And when disengaged, I can feel the truck go back to 2wd, but the vibration gets noticably worse for a minute or two, the abruptly stops all together. So whatever it is would be a part that spins while 4wd is engaged, and then keeps spinning for a bit after taken out of 4wd, then stops spinning altogether for normal 2wd operation. What is the most likely part to do this?

I would have t give the t-case a thorough checking out as it would still be engaged as you went out of 2wd. I would think it would have to be something with the 4wd part of the case and like i said before check for up and down and side to side play on the outputshaft as it can be a bearing or bushing causing this. As you don't have the mileage down i have no idea of how this is driven or used for. Of course it could be the frontaxle as well as power is driven to it and out. a good thing to do would be to check to see if there is any frontaxle dope in it as if not i'd beleive the bearings in that or gears could be shot . Especially the input bearing on the axle.