I would first change fuels. If the engine still pings after trying different fuel octane ratings and different brands, look closely at the knock sensor. The knock sensor is pretty easy to find it's just bolted to the engine block. It is basically a microphone tuned to hear things in a certain frequency range. Try to get this handled quickly. Pinging engines are not good. Things can break deep inside the engine (pistons, rings, and other important stuff).
My own taste for the hops is very powerful, and I had no intention of spending a beerless weekend...
a common prob with pinging is egr valve not working, it will cause a lean condition, as for knock sensor, more than likely it would set a code/check engine light would be on the knock sensor has a crystal that under vibration, it creates a voltage and the ecm will adjust timing acordingly. To check egr, it is the thing withg the tube going from exhaust manifold to a circular usually silver valve, it will have a vacuum line attached to check if it works either apply vacuum to the valve or you might be able to push the valve up with a small screw driver but be careful not to puncture the valve, when you move the valve at idle it should kill the engine or almost kill it, if all is well check that you have vacuum to i off idle i am not sure exactly what egr system is on the 94 but you get the jist of it