Hi, welcome to JustAnswer!. This is Ed.
You're right about the intake plenum effect. A gasket at the base of the plenum, called the "belly pan", can begin leaking, which will draw in crankcase vapors and oil spray, burning the oil in the engine. Deposits left from oil burning accumulate in the combustion chambers, increasing compression ratio and can become a hindrance to proper swirl and quenching effects built into the chambers, any of which will increase chances of detonation.
To tell if you have a case of bellypan-itis, clamp off your PCV hose on the passenger side valve cover. Start the engine and block the large air exchange hose that leads from the other side valve cover to the air cleaner with your hand. After running 30 seconds or more, a pressure differential should build within the engine from combustion bypass gases accumulating, so you should have a positive pressure woosh as you unblock the hose. If you have no woosh at all, or if you've built vacuum that goes woosh the other way (flow goes into the engine), it positively shows a vacuum leak exists below the intake manifold somewhere. It might be the bellypan or an intake gasket, but either one will produce similar results with oil ingestion over time.
If you notice no discernible pressure differential, you still have a leak. Combustion bypass gases are being produced at the same rate the vacuum leak is taking them in, keeping crankcase pressure at an equilibrium by pure chance. Piston engines always, always have blowby gases sneaking past the piston rings.
Other things to look for when ping is a problem include these...
Plug wire routing. Inductive cross-fire occurs when two high-tension plug wires lie next to each other. The electromagnetic wave produced around a plug wire (or coil wire) as it's fired will overlap into the adjacent wire, inducing an electrical potential in the second wire. Given the right circumstances, this ill-timed induction can be enough to produce a spark in the neighboring wire that can light another cylinder far too early, actually pressing the piston backward. The effect is greatest between cylinders 5 and 7 (back two on driver's side) and cylinders 4 and 8 on the right. These holes fire consecutively, so an inductive kick from 5 to 7 will or from 8 to 4 will fire the second cylinder far too early. Coil wires routed along the right side valve cover have the opportunity to affect all four cylinders on that side.
The cure isn't pretty, but effective. Route the 5-7 and 4-8 plug wires so they have as little contact area to one another as possible, crossing at right angles at most. The coil wire should be routed inside the area between the right valve cover and intake manifold.
Engine coolant chemistry plays a role, believe it or not. Too much antifreeze compared to water will reduce the capacity of the coolant mix to carry off waste heat, leading to hot spots near the combustion chambers. A 50-50 mix is recommended when ambient temperatures aren't expected to drop below -20 degrees F.
Too little antifreeze may allow cooling system deposits to build up between dissimilar metal types, like that of the coolant passages in the head gaskets and cylinder heads and block. Low flow in those areas will allow temperatures to rise in times of high demand simply because coolant can't flow at the rate it should.
PCM (engine controller) software was offered in 1999 for your model truck to reduce engine ping. If your PCM was flash-programmed with software from TSB 18-004-02, a software revision label should be visible somewhere underhood. Software flashes that happened after the release date of this particular flash will include the ping-reduction change whether you know or not, so any update label having a newer date than this TSB (like 18-xxx-03 or later) will have it on board.
Distributor adjustments aren't usually effective on these engines because it doesn't alter ignition timing. Crankshaft (or ignition) reference is taken from a fixed-in-place crankshaft position sensor, mounted above and to the passenger side of the transmission bellhousing. The sensor located inside the distributor is called the cam sensor and is used to synchronize injector firing. Rotating the distributor may increase the chances of crossfire within the distributor cap and can even cause some odd driveability symptoms as the relationship between cam and crank are disturbed. You're allowed 10 degrees of offset, plus or minus, between these two sensors before surging, difficulty starting or no-start develop. Time and miles driven will cause the cam-crank relationship to slip to the negative side as timing chain and drive gear wear accumulates, so if you haven't had your distributor synchronized lately -- like within 100,000 miles -- it might be time.
I'll stop there for now. Let me know if you have any questions and I'll be glad to help.