can I purchase 17" or larger 8 bolt wheels for this vehicle

Larger wheels and tires weight more and will increase the unsprung weight. Not only will this affect handling and ride, it will have a negative effect on fuel mileage. Larger tires and wheels almost always reduce fuel mileage.

The theory is that lots of people think, is that the bigger diameter wheel/tire will take you a little further for each turn, at same rpm of engine as before, and increase mpg. And it even might,at high speed, but only a miniscule amount, but is far out weighed for exactly the reason Bob said. It will only be worse, especially for the work you do with the truck, off road, your gear ratios will all be higher, including final drive, causing the engine to work harder to just get you moving