I would agree with the previous posts. Most of these things are always solved with a "simple" or "obvious" problem. Your tailgate being 1 or 2 inches too small is just not the usual answer. I would put a square in the bottom corners of the bed just in front of the tailgate. I would be willing to bed your bed sides are not square to the bottom. You could also check this by taking your measurement on top of the tailgate and then checking to see if that is the same as at the bottom. I would double/triple check before welding on some metal to make it work...chances are you have got it off by just a little.