Truck's sittin' a little crooked

[Cruisin' Oklahoma]

So, I've noticed when my truck is sitting on flat that it seems to sit up a little high at the back of the cab. The gap between the bed and the cab is smaller at bottom than at top by a little bit...I'm going to post one pic now and will post another tomorrow to get a better pic. I just can't exactly figure out the best way to fix a problem like this...Not a big deal, just gets on my nerves. I think it could be a body mount issue on it, what do you guys think?