You say oil is getting into your brake drums after a long drive? Easy fix. Of course change the seals at the backing plates and then remove the breather fitting on the top side of the axel tube. It's the one with a hose going up into the frame of the car. Take the fitting and drill the hole that goes thru it to 1/8"(.125) dia. The fitting has a very small hole from the factory and plugs up easy to cause pressure to build up in the rear end housing and cause the leaks. See I told you it was easy.
Another thing that I've seen happen lots (up here) are the Wasps that put a clay plug in the breather hose, a dead bug and an egg. Another clay seal, bug & egg etc etc till the end of the hose.
I've used 125 lbs of air pressure & it will not blow out.
If plugged like this, replace with a new hose & put a breather on the top end of the hose (they are made for this application) to prevent this.
What causes the leak is a build up of pressure inside the diff.