[84LX89GT]

Some things that can cause pinging/detonation:
poor quality fuel
overadvanced timing (or too much timing for gasoline)
plugged injector(s) - running lean in one or more cylinder
plugged fuel filter/low fuel pressure - not getting enough fuel volume under load
carbon buildup - decreases chamber volume increasing compression ratio/heat in combustion chamber, also carbon built up on back of valves can soak up fuel - mostly when cold and cause engine to go lean on one or more cylinders, ping under load.
at part throttle a faulty EGR system will cause the combustion temperatures to increase causing ping (going lean).

This is what i'd try:
change the fuel filter (if not done within last 15,000 miles) and change the grade of fuel to premium and see if there's any change. If it goes away with better fuel then i'd suspect either poor fuel quality or carbon buildup.

Good luck, hope this helps