Ken300D,
I got my 1982 300SD 3.07 rear differential from a local wrecking yard, Trolley Auto Parts, here in San Diego, for $200. It was used, and came with a 101 day warranty. I looked on the Web at another site in Sacramento, CA, and they wanted a total of $275 to ship a used one to me with a 6 month warranty. (Silver Star Recycling) Another local yard wanted $300 for one. The one I got was in excellent shape, and was clean, tight, and crisp.
I installed my rebuilt half-axles into the differential on a work table and I looked at and photographed all the internals I possibly could. (All I've ever seen were drawings and diagrams of the inside of a differential). Everything lined up crisply, so I then lifted the whole mess into place with a friend securing the diff bolts and inserting the half-axles into the wheel hub splines. As to the shimming process, I've seen reference to it on the MB disc, but it looked like it took a few more bits of knowledge, parts, and courage than I had available.
Looking back on it, I'm happy I replaced the diff and the half-axles at the same time, as it removed a major source of noise and worry about the rear end mechanics of my car. Next month I'm going to replace the rear wheel bearings as well. My car has 300k miles on it, and it is my daily driver, so I'm really just getting it ready for the next 300k! Hope this helps you...

SteveKaa