Replied to your original post:
http://broncozone.com/forums/index.php?showtopic=11325
For new rims, you want a minimum amount of backspacing of 3.5", but not more than 4.5", and 4" would be the limit for stuffing a larger tire than 33's. The reason is that a 33x12.50R15 tire on a 15x10" rim with 3.5" bs will rub slightly on the radius arms at full steering lock, and will rub the front bumper and front edge of the rear quarter panels without any trimming or lift. So, if you have a 6" suspension lift and 35x12.50R15 tires on that same 15x10" rim, you'll still rub the radius arms when turning. Some of the extended radius arms are bent inboard to give a bit more room, but I still rubbed on my 84 Bronco with the SkyJ 6" kit and longer arms. The farther your tires stick out, the sooner you'll rub something, JSM84