In this post I described some issues I am having with irregular shifting in my 300SD. The particulars of what is occurring don't seem to be important regarding my current question though, which is this... in that post I described having routed full vacuum straight to the modulator and not seeing any difference in my hard shifting. I have tested the modulator with a MityVac and it easily holds up to 15psi vacuum or more for extended periods. However, since my shifting is showing no improvement with or without vacuum to the modulator, I assume it needs either adjustment, repair, or outright replacing. What should I be looking for when I crawl under there? Is this a common symptom of a unit that is clogged up with oil and grime? Just wondering...

Could be a broken guide pin between the modulator and trans. Mine was broken when I removed my failed modulator. My failed modulator resulted in very soft shifts and it did not hold any vacuum. Beware when removing, fluid comes out.

Have you tried adjusting the little "T" lever under the modulator cap?

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