If the vent tube has been verified to be clear....
Yep! Good advice given above. Make sure the vent tube is 'clear' of obstructions, and the nipple on the diff. Your driveline guy should be able to blow through the tube, and the diff nipple.
Insects and varmits (or mud/debris) can build nests & plug the 'breathe tube'. A gob of 'goo' can plug the vent hole from the inside. When that happens, normal build-up of heat inside the differential creates higher air pressure. If the pressure cannot escape, then it will blow oil out of a seal. This is a commonly overlooked item and easily misinterpreted as a bad seal. Simple to test and remedy.
At least make sure he's inspected & crossed it off the list. If that's not it, then ...they're the driveline 'pro's' ...they ought to be able to figure out the root cause. As a mechanic you should inspect the seal one last time before you slide in the parts.
Some mechanics are terrible and should not do what they do. Even worse, terrible mechanics do the same WRONG things over and over again w/o noticing the little nuances which make their jobs go south continuously. Like slide axles over a new seal ...as they're putting the axle in. I can't tell you how many times I've seen that. Great way to ruin a new seal. Tons of youtube videos of guys doing that. You might ask for a different tech to do the work next time.
We used to have a lot of HVAC techs where I worked before I retired (I was an engineer). I've been in the industry for a long time and I can tell you we had a couple terrible techs who you could always count on to screw up a job. The rest were great (but the good techs know who the terrible techs are!!). Every company wants to say "our people are the best!" ...they aren't! Not all. Every company usually has at least one or two guys who should be sent out the door but sometimes it's hard to fire certain people. Fortunately for us, our new boss came in and 'cleaned house' of our terrible techs. It raises everyone's moral to know the guys who take short-cuts or don't educate themselves aren't getting a free-ride anymore. But guess what? Our old techs got new jobs at other HVAC places within a week! Prone to continue making the same stupid mistakes over and over again at another shop. Lousy guys are out there in every industry. Is that the issue you are running into?? I can't say. I would hope not. But almost sounds like they're not getting something right. It's not rocket surgery. I would ask the shop to have someone else supervise the repair next time.
But yeah, definitely make sure they didn't overlook a plugged vent tube....
