I'm gonna plead ignorance here - but what does the hose from the top of the differential housing to the frame rail do? And is all of that soup you drained out oil? There is no physical way it could be overfilled through the fill plug on the rear cover that much. Either the PO used the 'breather' (?) hose as a fill point or it's contaminated from water and garbage through that hose. And I'm pretty sure the fuel filter in the engine bay is OEM.