I have a '94 2.5L YJ (and the engine has almos 120K miles) with stock gears (4.10) and 31" tires, and on flat ground, I still have 5th gear... Unless driving against the wind, that is. Anyway, 31s and 4.10 or 4.11 gears is doable. For 33s you will need to regear for sure.