the prop shafts of the four wheel drive system have been replaced with new ones, balanced, and different shaft isolaters installed to prevent noise from coming up through the floor. tranny control module was recalibrated. shaft angles were also checked and an 1/8" shim was removed for shaft angle improvement. to tell you the truth, i don't really understand all that was written on the shop ticket, but the drive line (bad u-joint sound) has mostly disappeared. i feel there is still a problem as compared to the compititions all wheel drive offerings (audi, lexus, mercedes) that i drove. so be advised of the problem that GM has built into this model and decide for yourself. 85flhtc