For the original poster...remember, the wider the wheel and tires are, and the more stance you gain by a lower number of backspacing, the harder it is on your steering components...especially when you add the suspension lift. Focus on steering if you haven't done so already. I would do nothing short of 4.5" BS if you're going with 12.50" wide tires...and you can put a 15X7 rim with a 12.50" wide tire with no ill effects. Basically, your tire sidewall will bulge around the wheels more, but that actually helps protect things better like valve stems, hubs, and wheels. I don't really care about my wheels, as they're black steel and I can get them replaced for $35 each...but valve stems being ripped out and hubs being shattered on rocks can ruin a day on the trail (or at least make it a longer day then intended ).

My setup is on 35X12.50 tires, 15X7 AR767s, with 4.5" of backspacing...no rubbing on the frame. I could stand to go with a 15X8 with 4" of backspacing, but especially with 35's I don't want to go much more than that. Hope some of that info helped out.

Dave.

edit to add: If you're doing a 3+2 lift, you're raising your center of gravity obviously. With a wider track, you'll get better control. So you may want to go with a 4" backspacing after all to get that truck planted with less tendency to feel "tippy" on the trails. When you go up like that, it's usually a good idea to counter that COG change with going "out" as well. Wide track axles are always more stable off road, and you can mimic the effects of a wide track axle by pushing the tires and wheels out a little further.