Any luck? I have been chasing down a driveline vibration for sometime now as well. I have an SYE, Adams front/rear DS, and just got the gears redone. I have been chasing this problem since I lifted it. I have 6* shims as well. I only get vibes when going 50MPH+ and it gets progressively worse the faster I go from there. Also have a vibration on when letting of the gas completely at 50MPH+ speeds. I'm really at a loss as well, also just got gears redone 2K miles ago.

Still never found anything out. I did go to the 4x4 shop. The guy was extremely knowledgeable and friendly.

He took it on a test drive and instantly noticed the vibration. He looked through my entire driveline and didn't see any issues, and was pretty convinced the noise is somewhere in my rear axle assembly.

He told me to spin the rear pinion by hand and feel for rough spots or bad noises. I did that, found nothing wrong. His next suggestion was to throw on a 1" TC drop, even though I have an SYE. That would help me eliminate if it is an angle issue or not.

I have yet to add a transfer case drop, but honestly am kind of losing faith that I'll ever solve this issue

Don't give up. It's a process of elimination.

I'm chasing down a vibe in the front drive line. I started to pull the shaft and noticed lateral movement in the slip splines. My angles are good. Once the shaft was out the vibes are gone. U-joints feel fine but are stock with 112K on them. BTW, I cannot see the movement with the shaft out of the vehicle. Only when it was installed and secured on each end. Just had hernia surgery so my troubleshooting is on hold. There are many more things to check off before I know the worn splines are causing it.