Felpro makes a gasket, you can buy it at any autozone.  I have the number written down at home.  Gaskets and spark plugs and such are the only thing I buy at autozone. 
I always use the fiber gaskets and never ever ever had a leak on any rear dif this includes offroad vehicles.  The best part is the next time you take the cover off you don't have to scrape everything.  I think the guys who have leaks using them have covers that are bent.  Some of the dana difs came factory with silicone.  
FEL-PRO Part # RDS550281  $2.35 on rockauto.com