A couple notes for thought. The seals are a very robust design. I have changed out many of them due to leaking. I always check the vent tube to make sure I can blow air up from the bottom out the top, don't use a lot of pressure, 10-15 psi works for a check, too much will blow the dust cap off the top of the hose. If air won't go through the tube your case pressure will build up from the heating diff oil building pressure internally. The most common cause of the seal failing.
Second cause is the mechanic, they pop out the seals with a seal remover, sometimes that tool leaves a nice little scratch in the seal housing, the scratch makes a perfect leak path, you can install 10 new seals and the scratch will still allow a leak. Smooth the scratch with emory cloth or wet and dry sand paper, maybe 320 grit. Don't remove any extra material, just enough to smooth out the scratch surface. Clean with Laquer thinner or acetone. Put a thin film of RTV silicone in the scatch area to fill it. No extra, just enough to fill the scratch or groove. The seals have a rubber OD sealing surface already so tap it in and it is ready for use.
The last thing to do, the seal needs to be oiled before installing your axle shaft stub into the diff. Wipe the seal with oil, I use engine assembly lube just because I have it and it is thicker than the gear oil, lubricates better and stays put for assembly. I use Sealed Power LL5 assembly lube, it is good stuff, designed to protect on assembly and mix well with the oil once the car is run. Check the axle splines for any sharp points, smooth the end of the splines with the same emory cloth so there is not sharp points or edges to scratch your new seal as you install the stub. Oil up the axle stub splines and the seal surface on the shaft and install making sure you feel the stub shaft engage the splines before it clicks firmly into the case and the spring clip is engaged so the stub can't be pulled out easily.
I hate seeing repeat seal failures.
