Are the flex discs ok? When you removed the drive shaft, did you replace the center mount and bearing? I've seen center mount bearings cause weird noises & vibrations. Does the owner drive the car aggressivly(hard),maybe check rear hub & axles for "play".... I have a 500e customer who used to run the car on the track alot, he wore out one rear axle, and after I installed a renntech limited slip for him he started having problems with the other one at less than 80k miles.