Thanks very much. Apparently the drive motor is the same as the fan motor - about $200. I plan to take it apart and try to confirm your diagnosis before ordering the motor.
I did take it apart today, and found that with the linkage disconnected, I could hardly move the snorkel up and down myself - and I'm 6'-5". It was binding very badly - no wonder the motor couldn't operate it. I lubed the slides and sliding parts with WD-40, and it freed up nicely - now the motor does the job just fine. There wasn't any cooking grease - maybe the grease in the slides had dried up and stiffened over the years...
Earlier I had also talked to one local Thermador serviceman, and he too suspected the motor - said he'd never had to replace a slide.
Thanks again - I appreciate your basis of experience, and your answer was entirely appropriate without being able to see or touch the unit.