Been developing this lately (1991 W124 w 603.912), mainly left full lock, at first it feels like a brake binding because you first notice it when using full lock and reversing into a parking space.

Initially did a power steering fluid (ATF) change and new filter.

However, try the car in neutral and then park, swing the steering slowly from lock to lock. Listen to the pressure relief valve singing on full lock, a steady note = no air in the system.

If you still get the judder, try revving the engine, if the judder frequency is engine RPM related, and possibly disappears at 3k plus RPM, then the problem is NOT the power steering pump or power steering gear, it is worn engine mounts.

This will also cause the infamous serpentine belt damper to oscillate, even if fitted with a new replacement damper unit, creating the "worn serp belt damper rattle" common to all used W124 diesels.

This judder is also sensitive to front tyre pressure, due to the snow last week I was running 25 psi for better traction, now the snow is gone I am running the usual 34 psi for better economy, and immediately it improved the judder a little.

In the UK, OEM mounts from MB are £69 each, pattern parts from GSF are £25 each. Always do the pair.

I'll do this next week, tomorrow is MOT (annual vehicle inspection) day.