Before I tore my (56 F100) truck apart, when you looked at it from the rear it looked like one corner of the back of the bed was about 2" lower than the other side. The front was straight when viewed against the cab. I assumed the frame was tweaked, but when I got it stripped down, the frame looked very straight. Could what I saw be the result of a weak leaf spring? Is it common for one spring to "die" and the other stay pretty good? Maybe a load setting on one side of the bed for a long time?

 

It's possible,but check the spring pack for a broken leaf.That is more common.

 

Could have been the Bed also.
Remember these Trucks were once used as intended, WORK.

 

Springs are all intact. Any suggestion on how to test them for load bearing while off the truck (but still attached to the axle)?

 

I noticed the same thing on my truck, and thought the bed was tweaked or a spring was broken. As a matter of fact, both rear springs were broken, but that wasn't the problem. It was an illusion. Actually, the cab was crooked. The rear cab mounts (dog bones) allow the cab to pivot and mine had stuck off-angle. I put my floor jack under the low side of the cab and lifted it. It settled in fine. Now it all looks straight.
Example (before): https://www.ford-trucks.com/user_gal...=72200&width=0