I have an 08 CTT that has had a slight rear end vibration ever since I purchased it. The vibration is most prevalent from about 38-40mph and then again at 70+ mph though on some occasions, it seems to be minimal. The vehicle is lowered on lowering links (which I'm going to remove here soon and go stock with Durametric lowering) and has aftermarket 21" wheels with 20mm spacers in the rear. I also had a 4 wheel alignment done 10 months ago at the Benz/Porsche dealer (did not resolve it). There is definitely more negative camber than normal due to being lowered and on spacers. If the camber wasn't identical between wheels I would have assumed the dealer would have brought something up. I don't get too much vibration in the steering wheel either though I can still sense it.

Replaced the rear driveshaft about 6 months ago due to the carden shaft isolator blowing out. About 3 months ago I replaced both rear wheel bearings due to one starting to make some noise. I would have thought that either of those issues would have fixed it but it did not alter it. I know that one of the rear 21" wheels has very small hairline crack but it has zero issues holding air. The obvious thought was possibly one of the wheels got damaged in the back but I think I would constantly feel the vibration when the amount of vibration differs from day to day.

Does anyone have other ideas on what to check (other than buying new rims which I may end of doing here once the weather gets warmer) so I can get rid of this pesky vibration? Thanks!

 

Try rotating the rims from front to rear to see if it makes any difference. If there's no change in the vibration, you may be able to rule out the rims/tires.