Leaking from between the engine and gear box in Jeep Wagoneer?

As oldturkey said, it really depends on what kind of fluid is leaking. If it is motor oil, the rear main seal (large circular seal between the crankshaft and engine block) is probably leaking. If you have a manual transmission, it might be manual transmission oil (which usually smells like sulfur) coming from the transmission input shaft seal that seals between the splined transmission input shaft and the transmission housing. If you have an automatic transmission, it might be ATF coming from the torque converter or the input shaft seal on the transmission. ATF is red in color and feels more slippery than normal motor oil. Unfortunately to access all of those components, you must remove the transmission. Companies make "stop leak" type products that sometimes help to slow small leaks for both the engine and transmission.