Anyone know where I can get a new one of these? This is on the top of the axle tube and it's made out of plastic it looks like. This is on a 69 12 bolt posi rear end

Anything besides the plastic piece will work fine.

Sounds like you are describing the vent. It is plastic. It can be obtained directly from a GM dealer. Probably available through the aftermarket suppliers too. May be even NAPA. But the GM dealer has it for sure.

If you look close you can see it on this picture.

http://www.1969supersport.com/072.html

Rob