I'm running wheels with 4" backspacing and I had the same problem.. I had to cut a chunk out of the front lower corner of the bumper on both sides.
The smaller the backspacing, the farther out your wheels/tires will sit, and the farther out they sit, the larger the arc they travel in when you turn the steering wheel. Your tires are sitting about an 1" further outboard than stock (assuming you have 7" wide wheels, if you have 8" wide wheels then they're 2" farther out, and so on). Add to that you've gained 2" +/- in overall diameter and you've got a much bigger space that the tire is travelling in while turning.
2 solutions... trim the bumper, pound the pinch weld.... or get a body lift.