B2300 5 spd manual low power at highway speed

[NEFLAMOY]

Recently had a bad miss, mechanic replaced all plug wires and new coil. Miss is now gone, but truck has low power in 4th and 5th gear and will not run faster than 70 mph with pedal on floor! Runs poorer with AC on. AC fine at low speed but seems to drag truck at high speeds. Can failing AC cause these problems? Truck burns no oil and runs fine in lower gears, it's the high gear open highway driving that faulters.

[Tyranitar]

Misfires can damage and overheat the catalytic converter. You might have a partially plugged cat. However, before simply jumping to that conclusion, replace your fuel filter first. If the problem persists after replacing the fuel filter, take the truck to a muffler shop and have them test for backpressure.