I have a 97 f150 with a 4.6l and it has about 50 thousand miles on it. The only modifications are a Superchips chip and flowmaster dual exhaust. The engine seems to be significantly down on power and it pings when ever it is in overdrive. It also poped a code last week (misfire #5) and the engine seems to vibrate slightly under load and at idle. I also did a brake torque (with out spinning wheels) to see the extent of the pinging and the engine ran very rough under this load and even started back fireing. Here's what i've done to try and solve the problem:
professional fuel injection system cleaning including MAF and T.body cleaning
(the above did help a bit)
new fuel filter
new air cleaner
new plugs (NGK platinum)
new pcv valve
new wires

I also changed the thermostat when i did a head gasket a few months ago, and think it is faulty because the engine has a hard time staying as hot as it used to with old thermostat. Could this have some effect on the pinging.

next I am considering changing the O2 sensors and coil packs if you people think it might help. I am starting get frusturated with this problem so any suggestions that may help would be greatly appreciated.

 

Check the PCV hose assembly. They are prone to cracking, especially back behind the intake manifold. Could cause a lean condition which will effect the idle and pinging.

Have you checked actual fuel pressure to make sure that it is up to par?
Has the PCM ever been reflashed by the dealer? There were some updates to prevent pinging on the older trucks. You would have to remove the chip first.