Breather vent on D44

71-75 flanged axel 44's should have a vent nipple on the drivers side of the axel. The vent tube allows for the expansion of the air/hot hot vs. cold and helps keep the seals from sucking in water.
Could someone have attached a brake line mounting bracket to your hole?
you could drill and tap a hole

Most of the 44 rears I've seen vent through a special bolt in the brake line 'T' block. Typically this will be on one of the axle tubes and the other axle tube will have a brake line clamp bolted to it. Remove the bolt from the clamp and you can install a breather fitting there...had to do this with the Scout 44 in my project Jeep.

Dan, I had the same problem, someone changed the bolt to the t-block for the rear brakes. I ended up with a lot of oil leaks and water in the axle. I solved the problem by drilling and tapping another hole on the other side of the dif and incerting a 1/4 hose fitting, then ran the hose up into the interior under the seat and finishing it up with a fram fuel filter, which kills any smells coming from the axle, so far I haven't had a problem.(20.000 miles) Just make sure that you get all the metal shavings out!