just realised i haven't got one... hmmm

i can always weld/bolt something into the back cover.

its a atlas axle by the way, or at least the diff casing used to be... not much else is now

Yes. Gases inside expand as they get hot with sufficient pressure to blow your axle seals. Best get a breather on there.
Cheers

Is there not a hole for one already?

well the axle was re tubed, and i didn't think about iit. i've seen back covers with tubes coming out, so i guess i can weld a peice of tube or half of a steel tube connector in the back cover

If you do that then make sure it isn't where there will be oil splash or you'll just leak oil.

adrian