I had the gears replaced last year and since then I have changed the rear pinion seal 3 times! Now it will be the fourth. What is causing this? I had a shop re-install twice and the only conclusion they can came up with is that maybe there is something warped in the differential. I was thinking there might be something wrong with the yolk, like it was scratched or has some grooves in it.
Please let me know if anyone has ever had this issue.

 

I would make the shop stand by the work they did. Replacing the seal without checking the adjacent parts is just shotty work at best.

 

I agree. Funny thing, the shop that did the work just closed. I am SOL.

 

Maybe that's why they closed. But I would bite the bullet and take it to a well known shop that has made a name for themselves through their work

 

Either they can't install it correctly or the vent is blocked somehow.

 

I could not find a vent. That is what I thought too.

 

Assuming it is in the same place as SN95s it should be on top of the axle tube on the passenger side. little spring loaded cap. It should move freely and be clean.

I can see it on this axle for sale on ebay:
http://www.ebay.com/itm/1987-1993-Mustang-Gt-Cobra-5-0L-8-8-Bare-Rear-End-Housing-/180916323575?afsrc=1
That little nub sticking up on the passenger side tube a little bit right of the diff.

 

I see it thanks!