What does the floor of the box look like, have a look at your 2nd pic down. The u channel on the right looks like it is crushed somewhat and the bottom of it looks farther back than the top may just be the pic but it looks like it is kind of shaped like: \_\ rather than square. My buddies 90 Toy did the same thing but he had a body lift and used to fill the box up with firewood when we were camping and drive back to camp on rough roads which caused the back of the box to sag like that. On his you could see where the last section of channel that is the rear most mount had pushed a ridge up on the floor of the box. As somebody else said, if you bend the frame it will go the opposite way.