The tranny has been shifting rougher and rougher over past several months on my 300SD, and lately I hear louder and more frequent clunking from the differential case when I come to a stop.

Generally, the tranny shifts smoothly when it is stressed during harder acceleration, but on slower, more gradual acceleration, the shifts would be rougher (particularly first and last gears, and last gear would even clunk when not stressed). It also appears that having the AC on makes the differential case clunk louder when I come to a stop.

I am suspecting that it's more the vacuum gear-shifting mechanism and modulation than a bad flex coupling or something inside the differential case, but I don't really have any experience with these issues to say with any certainty. Do any of these symptoms point to a bad modulator valve or something related to that?

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84 300SD - bought in April '06 with 237,000 mi