GMC transmission issue

I assume you probably have the Allison? If so, in Tow haul mode the shifts can be a bit harsher. That is on purpose. T/H is designed to firm up the shift points and also hold them longer for better power when towing. When letting up on the throttle, and the transmission senses a downshift, like you tapped the brakes it will downshift kind of hard assuming your going down a hill and it needs to change gears to help maintain speed. I think your concern is normal transmission function and you just need to get used to it. If your dealer is good he scanned for any set codes on the TCM. Without any he is pretty much stuck. No one is going to pay for any non issue repairs.