I have a 2001 f150, 4.2L V6 with 120K miles. This week found oil is leaking between the transmission and the engine oil pan and not sure if the rear main seal needs to be replace or something else. I see that's engine oil and what would a dealer charge to repair?

 

Auto or 5spd?
Remove the inspection plate and see if you can get your eyes on that area. If your certain it is motor oil, it could be rear main seal or pan gasket leaking. Rear head gasket can leak oil onto the bell housing it but normally will run down the sides near the starter before you see it coming out that area. Plus that was more common on the 2v tritons.

 

I'm going to assume you didn't see the pic which has a transmission pan

To the OP, start by looking for leaks higher up and fix those, then clean up and drive the truck. You can also purchase a dye kit with a black light to see where the fresh oil is coming from.

I'm pretty sure all dealers are over $100/hr for labor and I would guess that to be a 5 or 6 hour job. I'd GUESS in the $700 range for a rear main seal leak.