Thanks for the response, Warren, but hindsight is 20:20.
The flex disk was hitting the tunnel only under very specific conditions. This led the tech to logically conclude it was symptomatic of some other undetected problem. (The fact that the situation coincided with the swapping-in of a new transmission only fed the assumption that something more complex was the root cause.)
You're conclusion about what I "should" have done is undoubtedly correct--and I'm kicking myself now for not having had the wisdom to foresee all of this. But, realistically, there were a lot of other things besides a defective new flex disk it could have been...
Sure do wish you'd served-up your valuable opinion earlier though!
Regards!