Before you start wasting money, "looks kinda flat" doesn't mean anything. The XJ springs ride basically flat by design, even a brand new one.
If you still have the factory flares, measure the ride height. For a stock suspension, the dimension from the center of the wheel cap (axle centerline) straight up to the bottom of the flare lip should be 17.0". If it's not less than that, your Jeep hasn't sagged.