Willrev,
About a third of the way down the page of this thread,
http://www.peachparts.com/shopforum/showthread.php?t=85165&highlight=2.47 a table should give you the info that you need on the various differentials that are compatible with your 82 300SD. Your car has the 3.07 ratio in it. I have an 84 300SD. I just converted it over to a 2.47 and couldn't be happier. Finding a differential should not be difficult. Evidently, they are pretty bulletproof and jillions were made for the various Mercedes models. Consequently, easy to find. If you get desperate try car-parts.com and input the necessary info. You may find something near you.
Good luck.
Tom