350 turbo will not shift to 3rd "drive"

I have a 350 turbo that will not shift to 3rd I have check vacuum checked the governor and it is free checked linkage what else could it be ??

Lip seals (usually direct) get clogged with sediment in those.
Sure, there are other causes, the most common is internal seals, especially if the other two gears feel fine.

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I had a T350 that wouldn't hit high. Found a tiny leak where one cooler line fitting wasn't tight, just barely dripped. Tighten that sucker up and it worked like new! My GM mech buddy said the pressures must have been marginal and that leak was enough to stop it engaging Drive. I dunno, sounded weird, but the fitting loosened off again a year later, I slid under and cranked it up tight and away she went! Go figure!