Shackle relocation brackets with longer shackles set in the middle setting(you'll have 3 positions), and bumpstop the crap out of it and you won't have that problem again. I've got a lot of weight in the back of my XJ in the form of a fullsize spare, tools, recovery equipment , spare parts etc and have never had my shackle hit the unibody even after launching the pig in the air and landing hard. Cheaper alternative would be to take the rear seat out and tell your 3 friends to get their own rigs.