Sounds like good advice. What if the dealer refuses to take the car back? My only recourse then would be to allow him to service the vehicle. I would lose if I took him to court, if he performs according to the terms of the Warranty.
Of course, these are all "what-ifs". I guess one step at a time. Tomorrow morning I'll know more. Then, it's decision time.
So, Onstar still reports only a code of C0306. They said they have the ability to read multiple codes, and that is the only code showing.
I think in the future, I'll advise anyone looking for a used Tahoe to check the transfer case fluid first!