Spark knock is pretty common on the early 4.0 OHV. Usual culprit is either 1) Carbon buildup in the cylinders or 2) A slightly lean mixture. 1) can usually be resolved using a solvent to remove carbon from the cylinders like Seafoam. 2) can be a little bit more challenging. A dirty MAF can cause the mixture to be a little lean. It's pretty easy to clean, so try cleaning the MAF filaments. A vacuum leak will also cause it to run lean. Make sure the air intake and vacuum hoses are all secure. One unusual vacuum leak is through the lower intake manifold gasket. It usually shows up as an internal oil or antifreeze leak around cylinder #5. Usually you can just tighten the eight or ten bolts and fix it, but occasionally the gasket is so far gone that it needs replacing.
Anyway, those things seem to get ~80-90% of people's pinging, so give those a try.
