2004 F-250 6.0 oil leak

As can be seen in the title, I have a 2004 F-250 6.0, with an oil leak :sick:. My mechanic has been working on it for some time and just can't seem to find it. He says it seems to be coming from the back of the engine, but only leaks after the engine is shut off. The catch is, it doesn't leak every time.

He has cleaned the back of the engine off, and added dye to the oil.

Any help is appreciated. Much thanks.