1/16/2023 - Here's an update. Above summary: Last winter they replaced (1) Torque Tube then (2) Drive Shaft under warranty. By the time driveshaft finally arrived, it was spring, so no way to determine if (2) fixed.
I now have 66K miles on my Blazer. We have not have bone-chiiling -20 F temps this winter where I've been out driving - so I wasn't able to see if this was fixed until a week ago. I was parked outside overnight in Northern WI for a below zero F night. When I started the Blazer next morning; it once again did the whine thing when moving.. however only for a very short time (~20 seconds) then all was normal.
I don't know if the two repair jobs helped any, but I decided I would not go back to the dealer unless we get another very cold snap when I can verify it's still a problem as they say there is nothing they recommend if they can't re-create the sound up on the rack.
My feeling is GM either (a) still doesn't know the root cause of this or (b) knows what the problem is and its can't be repaired or is very expensive. Either way it feels like they have their dealers throwing parts at it hoping the customer finally gives up.
I'm just worried this is not something that will result in major drive train issues when I get up over 100K miles.