Thanks Patrick, I forgot to mention that. Just think big spur and small pinion. As Patrick said, you not only have to accommodate the larger diameter of the tires, but also the added weight. You should expect much slower top speeds. You'll also need to pay attention to the steering. Everything is going to be put under A LOT more stress. I know the truck is light, but I'd go with at least a 100 oz/in steering servo and make sure that the servo saver isn't too tight. If you go with a servo that has a high current draw, you might also want to go with a bec that will send 6v to the servo.

I just want you to know that you are opening a can of worms. You're likely going to have problems that you aren't anticipating that aren't drive train associated. The larger diameter of the tires is also going to mean more leverage applied to all of the steering components. I can't even imagine what all of the extra unsprung weight is going to do to the handling.