I know this posting was from a while back, but did you ever fix that 'thunk'?
Just curious, because the note, "...adjusted Modulator" caught my eye.
I just recently "adjusted the modulator" on my '81 300TD-T to make my shifts a little more hard. It is very simple, and easy to find.
On the driver's side, just above the AFT pan, there is a 1/2 dollar sized (green on my car) modulator. Pop off the sm
There you will find a very small "T" shaped key. Turn the key 1/4 turn clockwise for harder shifts, 1/4 turn for softer shifts. This has nothing to do with the shift timing, only the softness/hardness.
Current: '91 300TE 4MATIC 317k and climbing...
'81 300TD Wagon 168K "Tank"
'83 240D 216K 4spd manual "Da Bear" (aka best car ever)
"Never sweat the petty things...
and never pet the sweaty things."