[OrrieG]

I would look very close at the frame rails just in front of the point it is only 1/8" difference. Either the right is bent down or the left bent up. You should see a slight buckle in the top of bottom flange of the frame. What happened? Could be rust weakening the frame at a cross member. could have had a heavy load dropped into one side of the bed. Could have been driven over a ditch or object with a load in the bed. Could have had the frame jacked to change a tire or get out of a ditch. You are probably going to have to pull the bed to get it back in place. A good old time frame alignment shop should be able to do it assuming you did not box the heck out of it. Think of that if you did box or weld on the frame that might have created a weak point or pulled the frame. Of you can secure the good side you might be able to jack up the lower side and see if anythings moves. My right front frame extension was tweaked a little bit and I used the same technique to level out my bumper when I first got my truck.

Also, looking at the old chevy spec manual I found out that the cross member connection that has a lower brace to the bottom of the frame at the end is called an "Alligator Jaw Attachment", says so right in the GM Manual!