Was checking the rear brakes yesterday and took a look at the axle vent.
It was stuck and didn't move so I sprayed it with brake cleaner and brushed any crap off with a toothbrush and it freed up and moves up and down when you push it with your finger.

Manual says if it moves up and down freely it's ok.
Has some kind of spring loaded jiggle cap on top.

No idea how long it was stuck but this original 8.8 diff has never leaked a drop so I guess it was venting.

Sound ok and good to go?