The term "missing out" is the best I could describe the issue. It feels like a "miss" on a gas engine. At 40-55 mph under a load (lugging up a hill, etc) it will "miss out" until you get above about 55-60 mph or until you accelerate harder.
The problem does not occur driving around town or at idle or when driving 70-80mph down the highway. It is only under a load between 40-55mph and it is intermittent, it doesn't happen every time.
I first noticed the issue after returning from a road trip to Tennessee and back which the truck did flawlessly. I dropped my girlfriend off and started on my way home. I had to slam on my brakes to avoid a deer and when I went to re-accelerate is when I noticed the problem.
The check engine light is not on. The truck has 263,000 miles.