It's not a matter of having enough backspacing, it's that the factory wheels have too much backspacing for 33x12.50 tires. The more backspacing, the more inside the wheelwell the wheel is positioned. The factory wheels have 5.25 to 5.5" of backspacing which positions a 12.5" wide tire in too much which makes it rub on the suspension.
You want wheels with a maximum of 4.5" of backspacing with 4" being even better. 3.75" is the least amount I would run however as less than that causes the tires to stick out too far.
4Jeepn, 3.75" of backspacing normally gives plenty of clearance for 12.5" wide tires, that's what I run. But if your inside sidewalls are rubbing, just place a washer or two under the steering bolt stop on the side opposite to the tire that is rubbing.