CRote and Robert Boyer,
Sorry, I missed your response and request for information nearly two months ago, CRote, but just in case you are still interested, here are a couple of suggestions.
I believe the differential fluid drain plug and fill plug are 14mm Allen head plugs, which means you will need a 14mm Allen wrench. These are available from a variety of sources, and I believe Sears now carries them. I bought mine in 1976 or so, from Snap-on, and it was about $15 or so then. The last time I bought Allen wrenches from Sears they were around the same price, a little less.
You need to get the car up on a lift, or jack stands, or ramps and jack stands to get enough room for you to get under the car and gain access. Always remove the fill plug first, as is you drain the fluid out, and the fill plug is stuck, so are you. So, the idea is to loosen the fill plug and remove it, then the drain plug. I think the plugs have tapered threads, and they can be very tight. I use a hammer to rap the wrench to get the plug to loosen. Works relatively easily, and avoids making room for breaker bars and excessive tooling and grunting. I will put them back in by hand then tap them about a quarter turn further to make sure they do not vibrate loose.
After the old stuff is all drained out, you put the drain plug back in, and, hopefully the car is pretty level, and you pump (I have a little hand pump that pumps the oil out of the container and into the differential) or pour or squeeze the containter to get the new oil into the fill port. You fill it unil it drains out the fill port, so the car being level has some influence on doing the job correctly. Then put the fill plug back in.
I use RedLine Oil Co. synthetic lubes for the transmissions and differentials. They are pricey, but they only get changed every couple years, and the RedLine products have worked very well in my experience. Go to www.redlineoil.com
to check what is available. In my 1986 190E 2.3-16 I recently developed a whine at around 65 mph, and the RedLine "Shockproof" fluid quieted it right down.
The rear shocks are available from many aftermarket stores, including FastLane, right on this site. Go to the top of the page and click on the "FastLane" tab, then click on the "on-line Catalog" button and the rest is pretty self explanatory. The windows will lead you to car and make, year, etc. then down to the part you are looking for, by its grouping (engine mechanical, engine electrical, body, suspension, etc.).
I have never changed shocks on a W126 body, but imagine they are similar to the W123 series. Post another question or do a searh on the subject. In general the job is pretty simple, a couple of bolts on the bottom, and a pair of concentric nuts on the top. Pay attention to how the parts came off, and get the sequence correct. The shocks come compressed, and have a tie wrap type restraint on them. New shocks are very stiff and you should not take the tie wrap off until the shock is in postion. You may have to soak the old bottom fasteners in WD-40 or the like to help loosen them. They live in a potentially corrosive environment and I have broken them off before, requiring me to drill out the tapped hole, and use a bolt and pair of nuts to lock the assembly. Major effect on how quickly the job gets done.
Once again, sorry about the delay, and I hope this helps, Jim