Because I'm going to need to label these things if they're diff-specific.

Reuse the shim on the side they were removed from. I guess that's diff specfic

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1981 240D 143k 4 spd manual -SOLD

2004 VW Jetta TDI 5 speed 300k -still driven daily

Well one axle's trashed, so is there any point in keeping it's shim?

Yes.
From what I have read the Old Shims seem to work on other Mercedes Axles most but not all of the time.
With the new made in China Axles this may not be the case.
However, there are different thicknesses available.

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84 300D, 82 Volvo 244Gl Diesel

That is true. I had to surface grind one of my original shims to fit the AA axles I bought.
MB did not offer a thin enough shim to fit the smaller gap

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1981 240D 143k 4 spd manual -SOLD

2004 VW Jetta TDI 5 speed 300k -still driven daily