This video is a tutorial on doing a BMW E36 Differential Fluid Change, its a rather simple, and effective way of extending the life of your rear differential and helping it run better. 

There are two bolts in the differential that deal with draining and refilling it, the bottom is the drain plug, and the top is the fill plug.

Tools needed are a 14 mm hex socket, 14 mm monkey wrench, oil pan, differential fluid (approx. 1.8 quarts), pump, jack and jackstand. 

First remove the fill plug, than remove the drain plug and drain the system. Once completely drained use a pump to pump the new differential fluid into the differential.