Toyota Repair: Engine Ping, engine management system, gas mileage

Question
I have a 99 toyota 4-runner, 3.4 v6, automatic, 150,000 miles.  The engine pings under load and acceleration. There are no other apparent problems or leaks, gas mileage is awesome @ 24 mpg.  I have done a full tune-up and have tried higher octane fuel and still no help.  What should I turn to next?

Answer
It seems that this may be caused by a lean condition, normally this will turn the check engine light on, is this the case? Any malfunction in the engine management system will turn the check engine light on, so it is puzzling to me how this can be with no check engine light.