If you do add or replace the rear diff fluid, DO NOT...I repeat...DO NOT USE SYNTHETIC OILS. These car's weren't designed to use them in engine, tranny, or diff. regular 15w-40 is the best oil to use in the engine at least. I would follow what the manual says to the letter. Synthetic oil might be the cause of the whining in the diff...
Anyways, on with the diff seal...
you might find this read interesting...
http://www.nmia.com/~dgnrg/page_23.htm