35R,
Rear Main Seal leaks are common on the 4.0 OHV Engine.
The original factory finish on the rear crankshaft snout seal surface was finished too slick, so the rubber seal can't hold back the oil.
Ford offers a "repair kit" that comes with a sleeve that has a rougher sealing surface that slips over the snout of the crankshaft, and a larger inner diameter rubber rear seal (p/n F5TZ-6701-A), but the kit costs about $60 at the dealership vs $20 for a "regular" seal.
I'm telling you this because last Christmas I had the clutch replaced on my X while I was off on vacation. While my mechanic was dropping the transmission, HE noticed the rear main seal leaking, and replaced it with a "regular" National Brand seal.
Two months later the new rear main seal it was leaking - to the point where there were 3-4 large drips collecting on the bottom of where the transmission bolts up with the engine, and 3-4 drips on the floor directly beneath...
Long story short - my mechanic provided the labor (under warranty) - and I provided the $60 Ford Repair Kit and a cold twelve pack of BASS Ale when I picked my X up from repair - and everything has been A-OK since.
HTH,
FD
PS - I do a lot of work on my X but this was one job that I was happy not to deal with - you've been warned - LOL!!!
