It reads like the pass rear spring is weak and/or has been deformed in its life.
Barring the option of removing the rear springs and checking unloaded height between the two and maybe even swap sides, the other item you might check is to measure the height of the bottom to the floor.
Let the car sit on its wheels on the blocks high enough so you can crawl underneath with a tape measure and measure the same two points the length of the car.
Make a ladder sheet, label front and rear, and measure a point on one side, and the same point on the other side. Then compare measurements.
You might even do this with a floor jack under the pass rear wheel to level the car, then measure underneath. If there is a twist in the body, this will find it.
If the car appears to be straight from the front to the rear while looking with the eye, it very well could be the flat spring. Check the rear spring front boxes for deformation. Check the rear frame rails to see if they are parallel and not one cocked up, this is where the rear spring attachs.
Everett "An Engineer's mind is never idle."