Clean the area as best you can (degreaser and power washer - avoid electrical connectors and components), then inspect the area after driving it some to determine the source of the leak. How high up is the oil on the back of the engine? Maybe include a picture from a little further away. Only thing that comes to mind back there would be the cylinder head gasket or valve cover gasket.
I am in the process of investigating this exact issue myself. I changed the valve cover gasket already, so know it wasn't just that. It did need to be done though. Gonna clean her up tomorrow and see what i find.