If the 1985 300SD is like the 300D, it uses a year-specific transmission. That makes them hard to find, but I saw one last month in a wrecking yard in San Jose CA. There was a thread on a different vehicle that covered various prices for reair in the Phoenix area:
http://www.peachparts.com/shopforum/showthread.php?t=107736&highlight=trans+valve

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'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!