Maybe a bad flex disk?

I know exacly what that is. It's the halfshafts going klunk when the transmission
downshifts and was given the wrong modulator pressure.

They pretty much all do that. Here's why.

The transmission shifting smoothness is controlled by the engine vacuum. See,
the smoother is shifts the more the clutch packs are slipping. Bad. But,
under most conditions the tranny can be told to shift fairly hard, like
under full acceleration.

Now, on a gas powered car engine vacuum is used; it's fairly straightforward
but on a diesel that has no manifold vacuum it's just not there.

So, there's a vacuum pump, and a bunch of vacuum circuitry to shape the
profile and modulate the vacuum as a function of throttle opeing. This is
then fed to the vacuum madulartor on the trans that controls the smoothness
of the shifts.

If any part of this system fails it's like having no vacuum going to the
trans vacuum modulator and the result is, if nothing else a major clunk
giong into second.

Good news: your tranny will last a very long time like this. Bad
news: your half shafts wont.

The part that usually breaks in this circuit is the little fingers
inside the black box on top of the valve cover. Though, a this
age all that nice plastic that makes up the vacuum control circuitry
has pretty much fossilized.

Good news: you can renew and calibrate all this junk yourself. Bad news:
you need some fancy gizmos and the vacuum lines are no longer available in
the original colors.

Lemme guess, your door locks don't work either. You can tell it's a Mercedes
when it klunks at the back end and the clue is the door locks don't work.

If they do work, congratulations our vacuum pump works. But, even if it
does, look for oil in the vacuum line, if there is any it's coming from
kill device and will take out the pump which will now need to be replaced
or rebuilt.

Complicated enough for you yet?

I replaces the kill device, replaced the vacuum pump, tested and replaced
all the vacuum lines and calibrated it. And changed the half shafts. It
took me 5 years to figure all this sh.t out.

Here's the vacuum calibration stuff:

http://articles.mbz.org/transmission/