I don't know if there is anyway to seal it completely but try using a heavier grease just in the areas where dirt could get in. The thicker grease will hang around longer and help prevent it from getting too far. If you spend a lot of time in really dusty areas then you want to clean and lube your diff more often.

I like putting in as much grease as will fit when I reassemble the diff. However, I don't think you need a lot. Just enough to coat everything.

Duneslider