You see that on a lot of old Chevy trucks. It is probley one of two things. Either the springs are old and sagging, or, the bushings are shot. Check the bushings first as they are easy to replace, and, not expensive. You may be able to check your springs by laying a straight edge across the top and measuring the distance from the straight edge down to the bottom of the spring at the center. Where they mount to the axle. Compare the left and right sides. If it is the springs you can either buy a new set, or, I think you can have the old ones "re-curved". If it is not the supsension you may check fo a bent frame or shackle problem.