How much do you want to spend? a 2" budget boost and 2' bigger tires will get you a total of 4" for not much money. A 3" suspension lift will run you more, but is probably the better option. More expensive, but you'll get a better ride and better off-road articulation. Just Google "Jeep Grand Cherokee Lift Kits", and you'll be amazed at the variety and price differences.
