U joints were replaced hoping that the vib would go away. But I can grab the drive shaft and it doesn't move. I have never touched the pinion angle. The PO did the lift and had the shims installed. Drive line is 100% stock other then gears, lift, and built trans so not sure what you mean by single u joint on each end. If its there factory its still the same.
Ok, so let's start at the back and work forward. Are the tires in balance? Are they round (not a stupid question, seen it twice where the tire had chopping and spent too much time chasing other issues to rule them out.) Is anything missing from the brakes? Are the axle seals leaking?
These are stated simply to help, I always start at the outside and work in when chasing a gripe, cause the parts are cheaper on the outside, and 90% of the time people don't shotgun those parts.
If everything else checks good, I would recommend removing the axle shim.
With everything else discussed, I would start there. The front and rear U joints should have their angles matching as close as possible. I don't know if your shims are welded in or bolted in, but if they are not hard to remove, should be easy to check.
I am still learning the specifics of duramax's, the advice I am giving is general, and how I would approach this issue if it was mine.
