Finally think I solved my vibration issue
Go to a friendly GM service department and speak to the service manager.
Tell him that you would like to take a test drive with their Smart EVA (Electronic Vibration Analyzer).
Whatever is vibrating will be reported in a certain Hz. That will tell you what type of component that
is causing the problem (Ex. a wheel will register at a different Hz than the driveshaft, etc).
It shouldn't cost much (if anything).