Greetings. I noticed during an inspection some fluid leaking from the top of my differential. There is what looks like a fill tube or vent sticking out of the top which was caked with grime. I cleaned it off and cleaned around the bottom of it...I ask what causes a leak from this area and what should I do to fix it? I have included a photo and the tube in question is visible in the center of the frame at the top in the cutout in the steel mounting plate. Thanks all in advance for a response.

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Marty D.

2013 C300 4Matic
1984 BMW 733i
2013 Lincoln MKz