Check for a bent tie rod. That will cause the toe to be out also. The reason one is wearing more than the ohter is that you are copensating for the toe being out along with compensating for road crown when you drive. That puts more stress on the left tire on a straight axle. Joe's method is a good one to follow to get a baseline toe setting. You will want ot make sure that the tie rod ends and the drag link ends are adjusted properly, along with the steering box itself.
