Minor jerking, not engine-related!

Hello TDI experts,

I've been experiencing a slight vibration (very low frequency) for a while now, especially at speeds around 70 km/h in 5th gear. Walkway noticed.
I always thought it was the engine, which tends to stutter at this low RPM and with such a light load.
However, I was proven wrong, because the phenomenon also occurs when I engage the clutch at that speed.
I can actually rule out a tire imbalance as the cause, because the phenomenon persists even after swapping the tires between the front and rear axles.

The slight jerking also occurs at higher speeds, and also when the clutch is fully depressed. What could that be?

The brakes should be functioning properly, at least after a brief check of the rim temperature following a longer ride.

"During the last tire change, I noticed a small amount of damage to the right (long) driveshaft. It's heavily rusted in a larger area. It appears that a stone impact damaged the surface some time ago, and rust has spread since then. I observed that the rust isn't just on the surface, but that the steel itself has already flaked off in several layers (so it's not just the paint)." Could this be the cause of my "stuttering" or "jerky movements"?

Does anyone have any other ideas?

Regards,
Alex.