In order for the shocks to smooth the ride, the suspension has to flex, at least a little. It sounds like you've got way too much spring for the weight of the back end of the vehicle and it's just jumping over the bumps. Try this- - - -put about 750 pounds of something in the bed, concrete blocks, steel, bricks, anything that will make the springs flex, and see if the ride improves. If it does, reduce the spring rate a little and you'll probably get a much smoother ride when it's unloaded.
Jerry
