Update - problem NOT resolved
Well, within a day of coming back from service, the problem reappeared.
I took it back in today and requested to talk to the mechanic and go on the test-drive with him to make sure he is hearing/feeling the problem I'm reporting. The shop foreman came out, we went for a ride in the parking lot and in about 30 seconds he reproduced the problem and said "wow, yeah, I got it"
He asked what they had fixed last week. I said I did not know exactly, all the repair ticket said was "resecured support under dash area". He said "didn't they replace the intermediate steering shaft?".
We went back to the office and he looked at the repair ticket from last week. He was not real happy that the tech had entered NO detail on the repair.
He then went to the GM website (dealer/service restricted area) and typed in my VIN and "KLUNK THUMP" as search terms. About 5 hits came back, the first describing in accurate detail the symptoms I had reported. There was a TSB to replace the "intermediate steering shaft" (something like that).
Now, I wonder why the service tech didn't do that to begin with?
So I will see later today when I get it back. I'll post the exact repair details in case anyone else has the same problem.