Like Chopt50wgn says, the PO went to a lot of expense to narrow the differential and tub the bed for those wide wheels & tires. I expect that to put on what you're calling ProTouring tires you'll be looking at replacing the rear differential with one that's not been narrowed so severely. This is one of those design features that's worked out very, very early in the planning stage of the project, and changing is likely not going to be easy, or cheap. The only way to know is to take off a wheel & tire and start measuring backspace/offset then get on the horn with the wheel guys. Good luck with your quest.