Loosen every bolt in the rear end (both ends of the leaf spring, and the shackle) one half turn, and then drive over some bumps fast. See if it levels out.
Sometimes if you tighten the bolts too much, or with the weight of the rig NOT on the leafs, one side (or both) will bind up.
After you've driven on it for a day, check to make sure the bolts are still snug, and then again a couple days later, etc.