Take a jack and support the car on a thin block of wood under the center of the front cross-member (car needs to be able to rock easily). Go and bounce the rear body gently. Did the rear level out? If so, then the front springs are pushing up unevenly (or you have a coil out of its pocket).
If not then either the leaf springs are binding or worn un-evenly. You can loosen the bushing bolts (eye bushing and shackles). try the bounce again. if that does not do it, swap the leafs side to side. Did the lean follow to the other side? If so, that is the worn leaf.