what causes BAD wheel hop in reverse

What you are feeling is the rearend twisting down and the leafs bouncing it back up, over and over again. Anything that can cause the drivetrain to start bouncing. Its most likely the first two but here are some other causes

1.incorrect pinion angle-adjust pinion angle with shims to 4
2.weak factory springs- switch to xhd or super stock springs for better forward traction and less bounce
3.clutch that has a weak center section and/or springs to connect the hub to the disc (cheap parts store clutches) also a performance clutch that grabs very hard due to a increased pressure plate clamping force
4.Loose or worn trans mount-even some cheap rubber trans mounts move to much IMO I would switch to a poly mount
5.low gearing makes this problem more noticable
6.warped flywheel
7.flooring it [-X