[f5fstop]

Your third gear theory is wrong, 4L transmissions will lock in manual 3rd gear, as well as in overdrive 4th gear.
Possibly an engine compression problem, but if it is the cylinder head issue, it would cause a rough idle, and the PCM would pick up a misfire after certain counts and turn on the idiot light.
As for buy backs, is this your first problem? Call your lease company. But if it natural, it is between your lease company and GM.

I really don't know of any heavy vehicle with an automatic and limited HP that will not do some shifting around on mountain roads. But then again, I don't know if it is a shifting problem or a TCC problem, or something entirely different. Good luck.