Light Load Pinging

[Nelso90]

I have a 1998 Ranger, about 144K on the clock. Some background on the truck, it has a K&N filter, air silencer delete, header back exhaust with MIL eliminator, and it has new plugs, wires, and timing belt. That's pretty much it engine wise.
The issue I'm having is that under normal around town operation, it's totally fine. Get it on the highway, about 2600 revs and 65MPH, and that's when the fun starts. Cruising is mostly fine, but when you tip into it slightly, maybe 1/8-1/4 throttle, it pings. Move past that throttle, and it stops, stay at that setting, it keeps on pinging. I am at wits end with this. I tested pressure drop on each injector for a given pulse width, and they all were even, the MAF was just cleaned, I gave it the Seafoam in the vacuum line treatment just before I took it out (still pinged). The only other things I can think of is either my pre-cat O2 sensor is messed up, the TPS is bad (hence why it is only light throttle), or the EGR is acting up (did some searching on here).
Anyone have ideas, I've run out!

 

[srteach]

When I had my 2.3, the pinging was eliminated with an O2 sensor change out. Not sayin it is your issue, but it worked for me.

 

[chilsam]

Sea foam

Give that a shot could have some carbon build up "DIESELING".

 

[Nelso90]

Before I get into a new o2 sensor, I plan on hooking up our school's Genesys to the truck and running a highway drive cycle, and watching some traces for EGR, o2 sensor, TPS and Knock sensor feeds.
One question, does this truck have a knock sensor, and if it does, could that be bad, causing the ECU to not pull timing when it should be doing so?