My pinion seal has been leaking lately so I used the magical search button and YT pulled through again. I suspect my breather is plugged. I took it out today and when I pulled it out of the hole it went psssssssss as the pressure was relieved that was in the diff. I cleaned the breather up and jammed a piece of wire in there and rooted around and then hit with the air hose but I still can't blow through it. Planning on getting a new one tomorrow but my question is this.......... If this was the culprit of my leaky pinion seal will it stop once the diff stops building up pressure or is the seal shot now that oil has been forced through it? Also I just had my oil changed when I regeared about 8K ago should I drain the diff and put fresh oil in it? I used some pretty spendy oil and don't really want to drain it if I don't have to. I did check the level the other day after I noticed the leak and it took about 1/2 a quart to top it back off.

 

Not uncommon to find a slight vacuum when unscrewing the breather, hot air is let out in use but once the vehicle stops, the breather seals up (as it is designed to do) and a vacuum is created as the air inside the axle cools. If pressure had built up, it would only be when hot, most likely.
Instead of replacing the breather with another stock one, try a raised vent line:

http://www.4crawler.com/4x4/CheapTri...html#Breathers

 

After I fixed my breather it stopped leaking, but I caught it pretty quick. Yes, you should change the oil after new gears are installed. That's why I put in cheap stuff for the first 1000 miles.