Look strait down at both sides of your rear bumper where the bumper pad goes slightly under the tailgate. Between the pad and the part of your box side that the tailgate hinges bolt to, you should be able to stick your finger in the gap and compare it to the other side. If its tighter on the right and wider on the left, its your bed that is off. Which means that cool rack will have to come off, take the bedliner out cause there is 6 bolts holding your bed down from the top. Your gunna need a T55 torx bit and a 1/2" impact with some guts to get them off. If they are frozen heat up 1 corner of the nuts on your frame, and if that doesn't work you have to grind the heads off and replace them and the nuts (Ford calls them 1 time use bolts). If the gaps are even then you have a side-sway or diamond frame condition. A few hours on the frame machine and your golden. Or you could just drive it the way it is, lots cheaper.
