It isn't the transmission! Forget the Avalanche. The mechanic called me to tell me that the van did what I was describing long enough for them to find a problem, which was the crankshaft position sensor and wiring harness. It ran too close to the exhaust to work on it today (too hot, no more day left) so they'll have it until tomorrow. BUT since it's just another $250 to fix we'll do that instead of buying the expensive truck with bad mileage.