Do you jack the truck from the rear differential? Anyone who works on the truck do it? Over time this can tear the sealant and allow a leak. I used to use brake cleaner on the cover to clean it off. That also chews up the silicone.

if you use silicone instead of a gasket allow 10 minutes for the silicone to set up before installing the cover. If you push the bottom bolt through the cover and just start the bolt the cover will rest on this bolt and lean away from the differential. Then do a top bolt and it will decrease the amount of smudge of the silicone and promote a better seal.

 

When i changed mine 3 years ago, i guess i didn't let it set long enough. I found it leaking the other day... got it changed today. They said they did use the gasket.