rough running, check engine light on.
Well there you go, the check engine light is telling you the truck engine needs service. After all it’s 20 years old.
A lot of things or combination of things can cause it to run rough. These things need to be replaced in intervals (every so many miles): Oil and filter, coolant, air filter, spark plugs, spark plug wires, fuel filter.
The timing (belt) marks being off can cause it to run rough. Any cracks or loose connection in any vacuum hose can cause issues
A check engine light will give you a place to start. Get an OBDII reader online for $15 or at a store for $20. It plugs right in. Obtain all the error codes and go from there.