30-31, is this auto or stick? IF you go too big then you'll have gearing issues...if auto then you can swap out standard gears for not too much money. These rigs are very capable even in stock form so you don't need huge tires.
Put some 30/31 on it and lock the rear and you'll see an amazing difference. If you lock the front then it'll be a tank. Remeber to strap the front pot because this is a bad weak link on these vehicles. If you can find some spare housing get them, you'll need them.