Power Train Driveline problems of the 1996 Buick Park Avenue

Vehicle strongly vibrates at speeds of 65 and more. Apparently loosened rf (passenger) door that was repaired by dealer. Vibration causes seatbelts to noticably vibrate, vibrations are sufficiently severe to travel through the seats to passengers, owner is afraid that continued vibrations will loosen body parts to result in a death causing accident. Note, mileage when purchased on 11-04-98 was about 5400, a gmac lease return to selling dealer. The vibrations occur each time vehicle exceeds 65 mph. Service department at royal Buick indicated that this vibration problem was not uncommon to 1996 Park Avenue vehicles.