I'd be looking up above first, and VERIFYING the source of the oil leak yourself. Many mechanics, friends, and good old Uncle Bob love to tell you it’s the rear main seal. Has a catastrophic ring to it, doesn’t it? A simple leak at the back of the valve cover or other source could produce the same symptoms. You don’t need to be a mechanic to figure this out. If you have good eyesight, you’re good to go on your own. Don't jump on the RMS/oil pan gasket bandwagon just yet.
Almost any oil leak on your 4.0 is gonna drip from the RMS area for two simple reasons.
First off, the engine sits nose-up and any oil will run back to the RMS area. Secondly, the RMS area is also the lowest point on the engine. Simple physics and the old plumber's adage apply here. "***** flows downhill".
Valve cover gasket, oil pressure sending unit, oil filter adapter seals and distributor gasket, in that order, have to be eliminated as possibilities first.