Thanks to everyone's help, as well as Phil @ PeachParts, I was able to find the correct part online and it should arrive by next Wednesday (3/23).

According to the site I purchased from, the part is called a control lever.

The part number is A6170703622 (Thanks again, Phil).

I called the dealer before looking around. Their price was $287!

I wound up paying $211 online, plus tax and shipping, bringing it to $250.

Still, an unbelievable price, for a part like this. Oh well.

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1979 300SD
1980 300TD 4-Speed
1984 300D Turbo CA