what causes BAD wheel hop in reverse

It might not be the rearend - you mention an auto to 4-spd conversion. Could be the clutch grabbing. I had a truck with a grabbing clutch that would buck pretty hard - but only in reverse & compound low. In the truck I could get it to back up smoothly by reving higher than needed and really feathering (slipping) the clutch in slowly.

That said - could be any of the things mentioned.