WTF is this port?

Starting preping my rear end for paint today (still in the car) and came across this long plastic "plug" sticking out of the top of passinger side of the differential. It's about an inch long, oddly shaped, and it pulls right out with your fingers. I don't believe it's a fill port, which I believe is located below the horizontal fin on the passenger side. Can anybody tell me what this is?
I can get pictures on in a bit if need be.





