1998 Chevy Truck Motor Problems

How long since a tune-up? Use a gage and check fuel pressure. Check for any applicable trouble codes. At the least, you need an obd2 code reader. Some of the national brand autostores used to check PO codes for free. You can call and ask.

You can use a pressure gage in place of the front 02 sensor. Raise the rpms to 2K or a little better, if the pressure reading exceeds 1.25psi, the exhaust is restricted, so says MOD.

First thing is, is the service engine soon light on?
First thing you need to do is have the codes read, Autozone does it free, Post the results with the code number(s). Even if the service engine light is not on, there should be freeze frame data stored.
If it won't make it to Autozone, buy a cheap scanner, they can be had for 30-40 dollars, a good investment, since they cover all cars sold in the USA from 96 on.
If the light is blinking, it means engine misfires possible damage to the Catalytic converter. Service it right away!