I agree with others here that it is probably a guide rod mount. Look at the rubber boot on the joint and if it is torn you should replace very soon. While you are checking this look at the ball joint boots and if they are torn replace them also. There are many right ups about this so search around. When I replaced the guide rod mounts on my car I had a friend use a long wrecking bar to pull the lower arm foward when I was starting to put it back together and it was no problem.

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1983 300SD
200000miles