Just wanted to give a final update and say thanks to everyone who helped out, especially those who suggested checking the trouble codes. I hooked up a scanner and right away found a fault with one of the transmission speed sensors. After investigating the issue, I discovered it was a common problem, so I ordered a new sensor and replaced it in the driveway. Once I cleared the code, the truck started shifting perfectly through all the gears and has been running great for the past couple of weeks. The advice saved me a lot of stress and probably a costly trip to the transmission shop.