Tachometer not working properly?

This can be caused by one of two things either the ignition system is back feeding into the tachometer circuit or the tachometer itself is going out. Can I ask if the engine is misfiring? If it has been a while, I would change out the spark plugs, cap, rotor and spark plug wires. Here is a generic guide to help walk you through the steps with instructions below for your truck.

https://www.2carpros.com/articles/how-to-change-spark-plugs

Check out the images (below). Please let us know what you find.