I would get an alignment done and make sure the caster angle is correct. I'm not sure what the stock caster angle should be, but a haynes manual or FSM would tell you, or a quick search of this forum would find it. At any rate, that's probably your culprit for the steering not returning to center.
You should also check the entire front suspension and steering components for loose/warn parts. Tighten any loose bolts. Grease all the joints that have zerk fittings, etc. Make sure the bolts that mount your sttering box to the body are tight. As far as I know, the gears in the steering box rarely go bad unless you have much larger tires. If you've just got 29's or 30's, the box itself shouldn't be a problem, but the mounting bolts do come loose often, and that can cause some slop.
But the majority of the problem is probably an alignment issue. Larger tires alone would not cause that.
Hope this helps - Rob
