The tachometer went out on my 98 Chevy C1500

Odds are the tach itself is bad. The driver signal for it comes from the engine PCM and is derived from the crank and cam signals recieved by the computer. If the vehicle is running and all else works, IE speedometer, cruise control, etc, then the tach is bad. I believe these were some of the first gen non repairable cluster units which need to be "exchanged" for a remanufactured unit.