I recently had a weird issue that after the truck was warm and we started hitting the hills, the truck would not downshift and we were going up the hills floored going 40 mph. Same issue happened on the way home, even manually downshiffting wouldn't let the truck go into passing gear. I brought the truck to my local transmission shop to get it checked out and also get a transmission cooler installed. Problem ended up being the throttle position sensor and it was not recognizing where the gas peddle was at and keeping the truck in a high gear even when it needed to downshift. The mechanic got it right on the first try. The truck is a 2000 chevy 2500 with 190000 miles on it, Still runs great and will use it til finally blows up. I am towing an 8000lb trailer with a Reese anti sway hitch.
Best of luck
