My differential was leaking slowly. After cleaning things up I saw the leak was coming from the fill plug. The culprit was the crush washer:
However, I noticed a slight sucking / slurping sound after I pulled the car up on ramps. I had just driven home from work, so the differential was warm and was now cooling. This made me suspect the vent. Sure enough, it was plugged solid.
Plugged vent on the left, working vent from my spare diff on the right. Even the non plugged vent was quite grubby. When you change the fluid I would recommend removing the vent and cleaning it.
Thanks,
-Jason