At this point, considering the various causes that we've seen, as well as the number where it is still doing it, I have a tendency to believe what they're telling you. I had read some time ago that one the possible causes was the lighter aluminum suspension parts, and other changes was the being looked in to. I haven't seen much on it since.
The following diagnosis might be helpful if the vehicle exhibits the symptom described in the PI.
A high speed shake or vibration that does not get corrected by performing service bulletin 00-03-10-007G.
Be sure to check the rear wheel alignment. The rear wheel toe and camber can be out of specification and create a vibration in the vehicle. The toe being outboard 1/16 of an inch can cause the tire to move outward then retract inboard at a rapid pace to create a vibration through out the body. This same condition can also occur if the camber is out of specification.
Please follow this diagnosis process thoroughly and complete each step. If the condition exhibited is resolved without completing every step, the remaining steps do not need to be performed. If these steps do not resolve the condition, please contact GM TAC for further diagnostic assistance.
(1998-2005 Buick Park Avenue, Park Avenue Ultra) and (2000-2005 Buick LeSabre) and (1998-2004 Cadillac Seville SLS, STS) and (2000-2005 Cadillac DeVille) and (2001-2003 Oldsmobile Aurora) and (2000-2005 Pontiac Bonneville )