You can loosen a couple of the bottom bolts and the fluid will drain out. I would prefer to take the cover off and have a peek inside. Then a new gasket or something similar and refill from the plug you see.
While you are there check your differential breather to make sure its not stuck. Its the little bump on top of the axle on the driver's side. It should move freely.


1949 Chevy 1/2-Ton
"Sedgewick"
In the Gallery
1989 Caprice