Well, it's been a while since I posed the question. Today I got the answer. I finally pulled the rear springs out and took them down to the spring service shop and had them re-arched. After a battle trying to get the outer sleeve out of the rear shackle, the reworked springs went back in without too much trouble.
After getting everything back in and the tires put back on, once I set it back on the ground everything straightened back up. The back end not only sat square again, but it also raised the back end about 2 inches. So, it was definitely old tired and worn-out springs causing the problem! Now time to move on to the front end.
