Gears. Rear gears can make the biggest single improvement in seat of the pants performance short of a forced induction setup. You'll e close on price, but I'd do that first. Then work your
suspension to get best advantage of the power you make.
The others suggesting exhaust aren't referring to car back. The biggest restriction in your ehaust is the places where you're necked down to a single. For best exhaust flow, you want duals aqll the way back, with a good x pipe. Cat backs only effect sound, do very Lytle for flow.
I don't know enough of the 4.0 to know what it's restrictions/weaknesses are, but I can assure you that a gear swap will give you the most noticeable performance improvement for that kind of money.