When trying to start the engine all I get is a high pitch whine, no sounds of cranking.

Other details:
This issue happened after the stop lever was held down while the engine was being started. Maybe a vacuum line broke? I'm stumped at this one as others have had no problem holding the stop lever while starting. I have access to a mityvac for testing vacuum lines.

Sounds like the starters drive is not actually engaging the flywheel. All you would hear is the starter motor itself turning rapidly if that where the case.

X2 on what Barry123400 said.
Or, the Overrunning Clutch behind the Starter Motor Gear is shot, or the Ring Gear us burred enough it is keeping the Starter Gear from engaging (metal to metal sound).

Get some more info ahd have someone try to start the Engine while you listen with a Mechanics Stethoscope or a substitute like a piece of wooden Broom Handle or large Screwdriver against your Ear.
Watch out for hot or moving parts!

Looks like the Starter needs to come off.

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84 300D, 82 Volvo 244Gl Diesel

Could that even happen when trying to start the engine in stop mode? Others have done exactly as I did with no ill effects.

I honestly do not know.
Could it be coincidence?

I also find that "trying to start the engine in the stop mode" an odd thing to try to do.
And, at this point do not understand why it would be done.

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84 300D, 82 Volvo 244Gl Diesel

Doesn't stop mode just manually cutoff the fuel? If the starter's throw out isn't working, you'd still get the spinning starter not connecting.

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Codifex
1981 240D ChinaBlue (Got her running with a donor engine.)
1983 300DTurbo w/sunroof.
1984 300TD manual sunroof. (Electrical Gremlins)