Originally Posted by Jona Gold

Just food for thought- reading your symptoms, I agree with others that said rotors, calipers, and wheel bearings are the most likely culprits.

But I have been down this road myself (chasing odd vibrations)- so I'll quickly replay what happened here:

Had swapped to a single mass flywheel clutch setup (2nd one in the truck) just like you just did. My issue was different from yours- it was only while the vehicle was NOT moving. At about 1300 and 1700 rpms, there was a very noticeable shake.

At the behest of the installer- I replaced the damper on the front of the crank (which made sense) with the upgraded 6.7L version, and I even swapped the rear drive-shaft, dropping the 2-piece and installing the 1-piece aluminum mopar "upgrade" because I also was noticing a new shudder on take-off. These things made a noticeable difference in smoothing things out, but did not eliminate the odd vibrations at stand still.

Reverting back to an OEM style DMF clutch kit was what finally eliminated my vibration issue. The Dual Mass reduces vibrations in the drive-line by design, and the G56 is designed to have a dual mass in place to do just that. I wouldn't rule out an out-of-balance brand new single mass flywheel. It's rare- but it happens, I was lucky enough to find proof. Also- those added vibrations didn't just cause an annoying "chatter" in my transmission. When I removed my transmission I found my input shaft seriously loose and had to pay to have my G56 get all new bearings, etc. (basically rebuilt).

Running a single mass- I would highly recommend you upgrade your damper (if you haven't already), and consider the one-piece aluminum shaft update as well. The more vibrations you can eliminate from your drive-line, the longer everything attached to it will last.

Originally Posted by rockcrawler304

Update

I removed the four bolts at the rear diff to change the orientation of the DS and found that u-joint was locked up in one direction!! Changed the u-joints and vibration is GONE!!

Originally Posted by seafish

NICE find….thanks for the update.

I'd venture that someone dropped a needle bearing inside that cap when reinstalling the joint and then just forced the cap and circlip on without knowing what they'd done…that would lock it up on one side and def cause a vibration!!

I always check for play and tightness in all directions after installing new ujoints…in fact, after a few "tighter" installs, I started using a bronze hammer to thwack the caps/yokes on each end AFTER installing the cir clips…the blow resets the caps and yokes just a little away from the spider and after doing so there is a noticeable gain in freely turning in what might be a tight turning joint (NOT that was YOUR problem…whoever did the ujoints owes you a least a six pack!!!)

Originally Posted by rockcrawler304

Actually the u-joints were factory! 135k miles.