if you have stock rims, up to 30" tires can be run with no rubbing and 31s can be run with minimal rubbing (usually just on the lower control arms and that can be fixed with WJ arms from the junkyard). if you have aftermarket rims, then the rules change slightly (eg: 31s will rub on the edges of the flares when turning).
don't forget that bigger tires will make your truck feel a bit more sluggish and will change your speedo readings (which can be fixed by swapping in the right speedo gear).