Haven't posted on this topic since September so here it is: Removed Flowmaster exhaust, removed 20" Denali wheels, replaced with stock 18's on Shi*ty Good Years. Vibration still there. Installed 2" drop shackles on rear vibration decreased some but still there. Going for rear sway bar kit next. Should be in this week will install and update. Also a small footnote, after removing aftermarket exhaust and wheel combo as dealer said still had the problem. I put the 20's with the bfg all terrain t/a's on my burban and no vibration, so its not the wheel tire combo. I would really like to run the 20" wheels but it looks like a no go. I am also having the pinion checked for preload after reading up on that some, as well as entertaining the thought of replacing axles and drive shaft to a steel 2 piece. Sounds like a parts replacement marathon right?
Dealer update: When I went through rounds 1,2, and 3 with the incompetent dealer service department they voided the warranty on my truck because of the aftermarket exhaust, wheels, and tires, they even went so far as to have GM customer service notify me of this as well. these as*holes really didn't want to fix my truck.