My guess would be a vent/breather, but I don’t know for sure. Those typically only leak when a differential is over filled.

Chorn said:
Not a good thread title/description, sorry. I have a lot of grime on one side of my diff. I see nothing obviously leaking, but the worst of it is near the top of the diff rear cover; there is something in between the two top bolts on the cover. Look here:

https://www.yukongear.com/DiffDetails.aspx?DiffID=139&PartsLink=PartCats.aspx%3FSearchMode%3DDiff%26TypeID%3D3%26Type%3DDana%26DiffID%3D139%26DiffName%3DDana+44HD%2C+Corvette+%26+Viper

Look above the Viper logo, to the top-right bolt hole, I'm talking about the dark greay/black thing sticking out next to that bolt hole. I think there must be a leak there, but what is that thing? I can't get close enough to mine to see it, but I can feel that it's some sort of nut that seems to spin freely. Is it a breather, or over-flow vent? What does it do?

Here’s an explanation. Different car but same thing.
http://www.miata.net/garage/reardiff/

Bigger pic of Viper diff attached below:
468_large_image.jpg

Thanks! Maybe it was over-filled last time the dealer changed it. I don’t look under there too often, so the leak/grime could have been there a while. Looks like the vent part is not available any longer, so I guess I’ll clean up the mess, then keep an eye on it, and if more appears, I’ll pull it out and take a look.