Not sure about Jeeps specifically but my Tundra has relatively heavy tires (315 Baja Champions), of course its Supercharged and modified a lot, but I still never use 4LO, (if you really want better response from heavy and large tires, it better to regear) it stresses the car out too much and I only use it if the car is really badly stuck. In general for most cars, use 4H, use 4LO only if you see the car isn't responding well or when you feel that it needs that extra push.