Notwithstanding the wheel issue, the OP does have aftermarket tires. Albeit, 265's but different nonetheless. Also, he has a lift. Who knows how the Castor is set after the lift. Then you look at the wheels/tires and it is not out of the range of possibility that his Castor is above 4.0. Not to say your "back spacing issue" is not a possiabilty, I am just thinking all things considered, it's just a matter of change.
Op, unless you are 300lbs plus, and most of that weight is in your gut, you have more then enough room (with a lift) to crawl under your rig and look at the inner "plastic" inner fender. There are 2 maybe 3 screws that hold it into place against the outer plastic fender. Take your hand and push against the fender, it will flex. Now will see which screws need to come out, then "re-drill" a new hole (after you push in the inner fender), and reset the screw. Very easy MOD.
Good luck and be safe....