Basically, it could be springs or your cab/box mounts. Best way to determine whether or not the springs are the culprits is to park the truck on a level surface and then measure from the floor to the frame, at the same point on the left and right sides at both front and back. If the left and right distances match, then your springs are ok (or at least are sagging the same amount) and you need to be looking at the cab and/or pickup box mounts.
