It is either the drop down where the tanks go, you have rear end separation or a cracked frame. For rear end separation step on the rear bumper and see if there is any movement between the shell and the frame. If there is any noticeable movement you have separation. For the cracked frame look between the wheels very carefully and see if the frame is cracked at that point. If it looks like a stiffening plate has been added you are fine.
FWIW I have a 1975 and mine has all three