Engine shut off while driving will not start

The engine was running fine for the previous 400 miles I drove today, other than some occasional pinging if I tried accelerating uphill on steep passes. When I refueled, I switched from regular to premium, as recommended for the 4.3l Vortec engine per the owners manual, and that problem went away. I started having troubles about 110 miles after filling up with Shell V-power.

I was just driving home through a pass in the mountains (Less than 1,500 feet maximum elevation, but some steep inclines) when the engine suddenly would not develop power. I could just rest my foot on the accelerator, and if I tried to put any more pressure on it, it would feel like I was riding the brakes and the engine started running rough. The issue only occurred when in gear. If I put the truck in neutral and stepped on the gas, it would rev up just fine. Unless I was going downhill, I could not exceed 45-50 mph. I would decelerate when driving uphill. About 30 miles later, the engine completely shut off and would crank, but not start.

The only lights I had occurred about 30 seconds after the engine completely quit running, and they were the battery and check gauges lights. Battery still had 12.2 volts when I put a meter on it, after about 6 attempts to start the engine again.

I am a certified aircraft mechanic, so I have understanding of how engines function, but mechanical issues with cars sometimes are beyond my realm of knowledge because the technology us far more advanced than what is found in most airplanes. Hence, why I am here.