I think with a load like that when the bed is weighed down and the cab is not it would make sense for the top of the bed to be closer to the cab then the bottom. There could be a lot of weight that is weighing the whole truck down so the cab is actually being pulled down a little too.
LOL that was funny . Don't bring a Ford haha. I've only had long beds but never hauled that much weight in the bed. I had a couple fords they had really bad side twist. This is my second Chevy but the first truck that I've hauled more than 1000 lbs with