Worn flex joints will let you turn one segment of the drive shaft relative to another. The clunk in this case is the slack being taken up as you accelerate or decelerate. There can be slack at any of the joints -- transmission to drive shaft, first driveshaft to second drive shaft, drive shaft to differential, inside differential, differential to alxes. The flex joints are rubber so you'd expect them to wear first.

I don't know how much wear is acceptable. As much as doesn't cause noise is the best I can guess.

There's nothing adjustable outside of playing with shims and preloads in the differential. You don't want to go there.

It shouldn't matter that you're using ramps instead of stands except that you can't spin the axles if the wheels aren't free to rotate. If your CV joints are intact, I'd focus on the flex joints.

Magnetism, gravity and inertia are all topics in Physics. And all topics I've forgotten by now

If there's lateral play in the driveshaft, indicative of a worn carrier bearing, you'll more likely get vibrations or a series of thumps against the transmission tunnel when you accelerate (I know too well ). It probably won't be one clunk.

You're lucky if you can see much of your driveshaft in a 124. In a 126 most of it's covered by a shield then further covered by the exhaust pipes.

Sixto
91 300SE
87 300SDL
83 300SD