I would think that without a body lift the 39.5 tires would rub off road. If your truck is under any kind of a warrenty you have to run the stock tire size, otherwise steering, driveline, suspension components will be voided. I have been running the stock size Parnelli Jones Dirt Grip ties for a while now, they are great on and off road, and still maintain a very agressive tread. Check out this website for more info on tires availible for the HUMMER:
http://www.amghummer.com/features/Tires/tires.htm