Ok so im having this problem and its driving me crazy!!, So here are the specs 33x12.5 goodyear duratracs on 15x8 steel rims i did the zuk mod with front yj springs replaced rear bushing with polyurethane and has longer shackles with procomp es3000 shocks. So when i get to 60mph the rear tires start bouncing and i basically fixed everything except for leafsprings (hence the zuk mod)

 

Just a guess but it sounds to me like your rear suspension is to stiff for the weight of the truck.

What happens if you put 300 or 400 pounds in the back