Please note: if you use a larger diameter tire on the rear (or a smaller diameter tire on the front), you will alter the caster angle in the front end--the angle will become more positive. While using a large diameter rear wheel is one way to lower your overall gear ratio, a more positive caster angle will always cause your truck to wander more. The closer you get to a caster angle of 0 the more you will wander, and re-shimming the front axle is the only way to remedy this.
Jon
