Rod going from under car behind wheels to control arm, just under spring is called brake corntrol rod, guide rod, support rod, or something else. Depends on who ya talk to. Bottom line is that the (ball) joints at the rear go bad and need to be replaced every 100k or so. While you are doing that, might as well do the subframe mounts that are in the same assembly as the rod joint. Check fastlane or the dealer of your choice. It is a DIY job of moderate difficulty.

http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1A80GYW8Q1G20VAJ9L&year=1983&make=MB&model=300-SD-002&category=L&part=Guide+Rod+Mount+Kit