Jeep Wrangler Forum : “Has anyone mounted 32x11.50 tires on factory 15x8 rims? If so, do they rub at fu...”

You can run 15x8's with anything as wide as a 12.50" tire.  I run 33x12.50x15" on 15x8's all the time.  However, the issue you run into with a factory rim would be the 5.5" backspacing for most Jeep applications.  If you absolutely are set on running the factory rims, order 2 sets (Sets are sold in pairs of 2) of Spidertrax Wheel Spacers here:

http://www.quadratec.com/products/92807_001.htm