If you want good tight steering with those big tires and that heavy truck, you will need a powerful digital servo, I would recomend the Hitec Hs5945 (.13sec & 183oz of torque @ 6volts) this servo has tons of holding power too, something like 300oz.

You can pick 1 up on ebay for about $85 shipped, or you can pick 1 up here for $89.99 + shipping. One of the best things about this servo is not only is it strong, fast and has all metal gear terrain, but its completely sealed from the eliments with 7 rubber seals.