I believe it has something to do with Torque twist. When reading about the TC3, before it come out, they (AE) talked about this, saying, the directional torque of the motor spinning would offset the directional torque of the shaft and diffs spinning. It was also to help torque steer, but I dont think that really ever had much effect on either. Ive had many of these (TC3,TC4) and they all suffered from this affliction. I still love the sound of a shaft drive though.....
Chad O