Here's everything I've done so far:

Replaced distributor cap, rotor, spark plug wires, spark plugs, timing belt, idler pulley, tensioner, ECT sensor, air filter, fuel filter, catalytic converter and I just got the injectors cleaned.

I've checked/adjusted to spec the TPS, timing, VAFM, VSVs and I've checked for vaccum leaks.

Gettting the injectors cleaned seemed to postpone the check engine light but it hasn't stopped it from coming back. It only comes on when I'm driving on the highway and it doesn't matter whether I'm under load.

The truck starts to run fairly rough and my initial idle is about 500 rpm after the engine warms up especially if I've turn it off then on a few times. Then the engine pings when I first accelerate and real bad if I'm revving at 3,000 rpm or so on the highway. It seems to also only ping if I hold my foot steady. If I let it off a bit it stops for a while and if I press it more it stops for a while.

I've read up on this issue and a lot of people say the O2 sensor. I've replaced the sensor but I got it from a site that wasn't particulary reputable. I don't even remember the site name. Plus it wasn't NGK or Denso which people have said are the only sensor that work alright with this truck. People have also said the fuel pressure regulator.

The O2 sensor is the last thing from the list of causes from the FSM. I'm just look for some second opinions.