I don't know what your driving other than a 5.0 Ford 4x4. But they way you described the shifting & saying you might try the manual, I'm thinking a 4 speed automatic, with 4th gear the overdrive. If that's the deal & your running in overdrive, Don't do that! Pull that slacker into 3 or whatever it's called. Want hurt a thing, & will save you a transmission! I learned an expensive lesson once, with a 93 K1500. Old 350 engine & 4l60 trans, towing my CJ on a trailer to Crossville. I had bought the truck used & wasn't that familiar with the trans. And what I thought was a trans cooler was the oil cooler. Almost made it to Crossville, before I roasted the transmission. I Did hold it in 3rd. on some hills, but not all. And with it shifting up & down, plus no external cooler, it simply overheated. Roasted that pig! Climb the hills in 3rd., & let it turn some RPMs. You might have more power than you thought. Same for coming down hill, keep it in 3rd. But up or down, don't Blow it up! Anywhere between 3000 & 4000, you'll be good! Closer to 3000 be better; mileage & engine. I'm speaking of Upper rpm limits!
Newer trucks have 6, 8, & even now, 10 speed automatics. You can't afford the 10 speed yet, but you might could get in a 6 or 8. Those keep the rpms at a more constant speed & torque, plus the engines have far more power & torque.