Diesels usually do shift at extremely low (IMO) RPM's. Thats the way the tranny is set up. (the Th400's in CUCV's shift 1-2 at about 10MPH IIRC for example)
Not sure exactly how much input the TV cable has on shift timing, and although I don't doubt that it has some affect, the governor is, as I understand it, the main factor in shift timing.
Sounds to me like perhaps the TV cable has been messed with. The gas TV cables (diesel should be darn near identical, GM is cheap) are pretty much all black plastic, no metal on them. They just "clip" into a bracket on the intake. "Universal" TV cables are regarded as junk by my friend that owns a transmission shop, as they apparently allow the cable to continually "adjust' itself. I'd think about buying a new cable from GM if you can afford it, so you can rule out the TV cable. (if you are truly worried that the cable is not correct) perhaps look around in wrecking yards and compare your cable to others. Another option would be to go to an auto parts store and compare, just make sure they aren't showing you "universal" TV cables either.
Dorian
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Why insist on counting when the ring gear has the tooth counts stamped in?