I am not certain about the 126 chassis bit on the 124's worn bushings on the rear control arms will make a pronounced clunk sound at shifts, esp 2 - 3 and 3 - 4.
The MB transmission is supposed to shift a bit hard, granted you can slew it by adjusting the vacuum modulator, that will increase wear on the clutches however. I've grown to just accept the harder shifts and replace all the rear control arms. The clunk then becomes a slight kick in the pants (or snap in the neck) as the tranny shifts.
So before you try to smooth the shifts out a lot be sure there aren't some suspension parts that are at the root of the noise.

__________________
'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!