So, I bought my 300 for its engine, to transplant into my 1984 4runner, which I have not done yet. Yet it sits in my yard, and it has a soul, and it is beautiful in it's way, and it has a fairly good body by non-California standards, and.. I have grown attached. When I drove it in the past, it had an knocking coming from the gear selector area. The trans worked fine other than that (after making the usual adjustments), I am wondering if this is something terminal, or an easy fix? Any thoughts?

If its slack in the driveshaft you will hear thumps during shifting ,clunks when putting into gear.The 2nd guess is it understands what you are about to do too it by tearing its soul out and putting it into a japaneze can.You may need a car whisperer to fully understand the 2nd guess.
Seriously though ,in most of these trannie noise issues a lift and a good tech with another person in the car for testing the noise out by having it in drive and reverse to gain its exact point of origin.

It may just need shifter bushings. I went through this in the SDL this past weekend. Worn bushings will cause a rattle, clatter, or knocking noise coming from below the shifter or from the shifter itself.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

I think the shifter bushings sounds right, although I'm not discounting the car whisperer theory, lol. Thanks

I no longer whisper to my car. It's a tough old bird, it can take some swearing. Thick German skin and all.

The shifter bushings can cure other issues as well. Shutting up the rattle is only one of the benefits. Having a nice firm shifter is the biggest plus, followed closely by no longer needing to fiddle with the shifter to restart the car when parking on a steep hill!

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Ditto... I've done this on three (3) cars (including my buddy Ed's). Memory may fail me but there are about 5 bushings... replace them all. I did buy a tool specifically for this... from (whoops can't mention the name ) but, it did help immensely.

__________________
'84 300D (211k) *New*... (Wife's)
'85 300TD (174K) 10th Year... (Mine)
'96 F-350 7.3L Dually (149K) Since New
'85 300D (156K) ~~~Wrecked~~~ Damn it! (parts)
'84 300D (176k) ~~~Wrecked~~~ Damn it! (parts)

Thanks for the advice, I will give a try when the weather improves. I would like to at least be able to sell the tranny with confidence, as it is a 1985 model and I guess somewhat rare, and works fine otherwise.