Here is a great How-To by some brilliant author that may lend a helping hand...
How-to Eliminate Pinging - SeaFoam Motor Treatment
Let me summarize what you should do, since it appears everyone is going to chime in with individual actions instead of a list.
Go to the parts store and buy:
Electrical Contact Cleaner (CRC makes some good stuff)
SeaFoam Motor Treatment
That should total about $15 or so...
Now go home and WHILE THE ENGINE IS STILL WARM, ingest the SeaFoam through the break booster vacuum supply line. This is a black rubber hose that runs from the intake manifold to the brake booster. Disconnect the end at the brake booster, and prepare an easy way to suck the SeaFoam into the motor using this tube. Some people pour the SeaFoam into a bowl, but I like to slip a smaller diameter tube into the brake booster tube and slide it into the neck of the SeaFoam bottle.
With the engine running, suck about 1/3 to 1/2 of the SeaFoam into the engine. It WILL sputter and maybe even stall during this process, but re-start it and keep going until the proper amount is in. (You may find that you have to plug the hose with your thumb to allow the engine to reach a steady idle.)
Now, turn the truck off and let the SeaFoam "soak". While it's sitting, disconnect your air intake tube from the throttle body. Remove the MAF or MAP sensor and spray the electrical contacts liberally with the electrical cleaner. Let the sensor drip-dry. Also, clean the IAT sensor or any other sensors along the intake tube. (sorry I'm not personally familiar with the 92 intake monitoring circuits)
After you get the intake put back together, continue to wait until the SeaFoam has soaked about 10-15 minutes. Be sure to put the injector cleaner in your fuel tank while you wait. Finally, RECONNECT THE BRAKE BOOSTER LINE, start the truck, and drive it around the block slowly. Make sure to be especially observant of all the people that will be pointing and laughing at the huge cloud of white smoke trailing behind you.
By cleaning these sensors, you have provided a more accurate reading to the PCM, and it will adjust the fuel injector on-time accordingly. By using the SeaFoam, you burned out built-up carbon in the intake, providing the engine with slightly more air and cleaner, more efficient combustion.
More air + More Fuel = Less preignition or "Pinging"