Check out the picture in this thread:

http://www.peachparts.com/shopforum/showthread.php?threadid=23471&highlight=differential

Looks like Mobil 1 will clean the inside of you diff without having to open the case. You might use Mobil 1 once as a flush for a month or so then change it with a long term fill.

Sixto
91 300SE
81 300SD