blazer leaking oil from back of engine

I have a 98 blazer that is leaking oil from the rear of the engine. The oil is running down the transmission cooler lines only when the truck is running at a steady drip. I cant see where it originates from but it appears to be coming from on top of the rear seal.
thanks EDIT EDIT EDIT Sorry for the confusion!!!!! after further inspection i found that the o2 sensor was rubbing the tranny cooler line. After moving it while running the engine I found fluid was spraying from this damage. Now the question is how do I repair??? thanks for the replies

With a mirror and strong light, check the oil pressure sending unit. It's on the block on the drivers side of the distributor.