[FormulaForce]

I'm no expert but I wonder if you twisted or bent the whole rear axle in it's mountings if the bottom of the differential case hit the bump. That would throw your driveshaft/rear-axle u-joint out of alignment in an up/down way, and put strain on the bearings. These cars are very sensitive to driveline alignment and balance issues. My 95 gets that "let off the gas" vibration at high speed even with a carbon driveshaft. That would be my guess if there are no dents in the differential housing that would affect the ring and pinion gears. As a WS6 your car should already have a limited-slip rear axle, so getting a used one wouldn't make any difference unless your housing is really damaged. You can get different gears for about $200 new (parts only), but they need to be installed by someone with experience or you risk bad gear meshing and whining. If you put in new differential fluid, you need to include limited-slip additive as well. Just my 2 cents worth, good luck.