Excessive Pinging

I have a 1994 Toyota 4 x 4 Pickup with a manual 5 speed, which has the 22RE engine in it. I just bought the pickup from my mother who has had the pickup since it had 15,000 miles it now has 110,000 with not a lick of trouble. The oil has been changed religously every 4000 miles with synthetic oil. It was my Dads thing he thought the synthetic oil was better then regular oil. But now that you have some history. The pickup pings bad when you get the engine warmed up after about 15 minutes of driving at 60 mph on a level flat freeway. It pings worse after I turn the air conditioner on. But even after backing off on the air conditioner it still pings. I have tried using higher octane gas to no avial pings just as bad as before. Any ideas what could be happening. I have heard of a knock control sensor as a possibility. This pinging does not sound good an I do not want to damage the engine for prolonged pinging. Any ideas would be appreciated.

Pinging in Northern Calif.

You sure what you're hearing isn't the valves?
When was the last time the valves were adjusted?
Valves noise is common to the 22R engine.

When was the last time it had a complete tune-up?
Check the timing, valves. Camshafts are known to wear causing excessive valve lash.

Welcome to the AF!

Try cleaning the carbon out of your combustion chambers with some Seafoam or GM top engine cleaner. With the engine running at normal operating temperature (in an open area away from houses if possible), slowly pour Seafoam or TEC into a vacuum line (like the brake booster line) and keep on the throttle to prevent your engine from stalling. Once a good portion of the can has been run through the engine this way, dump the remainder of the can in fast and allow it to kill your engine. Let the engine sit this way for a while and then drive it out on the expressway, downshifting occasionally to rev the engine and burn out the carbon and cleaner.

Incorrect ignition timing (as mentioned above) is also a possibility