Look around the back of the motor where it bolts to the transmission. Although the 4.0 motor is not as bad as the old 4.2s for rear main seals leaking, it is a known weak point. Also check the valve cover, those gaskets tend to get hard and let oil leak out. If it's the rear main, a bottle of seal conditioner (stop leak) in the crankcase will take care of it for a while. You will need to take care of it eventually. If it's the valve cover, you'll need to take it off an replace the gasket.
