This is what Rick is talking about. Here's the engine I'm putting in my 4400, it's from a 3500. The axle comes off with the engine and the seal is behind the flywheel. There was a few drops of oil on the seal so I pulled it out this afternoon. While I'm there, I'm replacing all the clutch parts and tran's seal too. I'd check the rear main for slop while your there. Hopefully it's not the cause of your leaking oil. Mine has .002 with me lifting for all I'm worth. Anything over .005 and the service manual says it's time to overhaul the engine.