I put my car up on jack stands, and with the calipers and
rotors out of the way it's pretty hard to rotate them by hand...
even when the manual transmission is in neutral. I checked
both parking brake shoes and linkages and they look fine.
I even readjusted the parking brake starwheel to make
sure there is no dragging there...

I am not sure if this is due to the normal friction from the
differential, axle components, etc. OR if something serious
requiring my attention is going on...

Is there a way I can measure this type of resistance with a
torque wrench (I remember reading an article on doing that,
but I haven't been able to locate it) and then compare that
to the range of values considered normal for my 1979 240D?

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1979 240D, W123, 105K miles, stick, white w/ tan interior.