what backspacing to run

You should check out the "Post your wheel and tire combo's" thread here...http://www.forabodiesonly.com/mopar/showthread.php?t=14254

It would also help to know if you're running stock suspension and brakes, or any upgrades you've made as it will make a big difference. Typically, 4.5" of backspace is good standard choice for 15" rims, larger rims can sometimes get away with more but that's usually only true for 17" and up.