Reverse bounces under load

[Hardball]

My 77 F100 400 2V C6 used to bump a lot backing up. The more load, the more bump. We replaced the 2.78 (I think) differential with a 3.31 (I think) limited slip. The spider gears were real worn. But it still bumps only a lot less. The C6 was rebuilt before the rear end change and then again to repair a broken clutch pack housing that the spline center snapped out during a heavy backup move. What else can make the reverse move go bump?

 

[Jolly Green]

U-joint? Did you replace those when you did the rear end?