I fit 285/75R16(33x11.5ish) on my stock 2500hd 4x. Had to trim the lower valance of the bumper a little but other than that no rubbing and I didn't have to crank the torsion bars. I'm sure you could fit the equivelent size tire in an 18. The 285/75 was a little wide for the stock rim. Just be sure to get as much back spacing as possible to tuck the wheel and avoid rubbing while turning.
Good luck,
Chris