Is this some kind of differential seal?

I found this circular piece of plastic flapping around at the junction of my drive shaft & differential. Does anyone know what it is?
I've got a slight leak from the diff I think because it's a bit oily around there, but there's no really obvious source. It's kept the steel in good order thankfully whatever it is

 

I would think it looks like the outer portion of the pinion seal

 

Avern, thanks for that.
I think I'll avoid rebuilding it right now but keep an eye on it.
Huge job from what I've read here.
Chris