If it has a transmission cooler you should check it and see if the fins a plugged with dirt/bugs. There is a sensor for transmission temperature and if it goes over a certain mark will send the truck into 3rd. Makes sense that after driving for 1000 miles with blocked air flow to the cooler that eventually the temp would creep up.

Just a suggestion.

Hose the fins down until you see clear water running out the bottom.

Hope it works, if anything it will cost you zero.