The NAPA listing is wrong, there is no knock sensor on any '96 4.0l. They were only used on '87-'90 XJ Renix 4.0l as Bigbird79's post said earlier.

On '96 4.0l engines, TSB 18-24-98 applies to reprogram (Reflash) the PCM to eliminate light spark knock under load. What this reprogramming does is allow the timing to be retarded up to 4 degrees from base timing using the DRB3 scan tool. High end aftermarket scan tools may be able to retard the timing, too after this flash update. Note: If your knock is heavy, the TSB recommends further diagnosis.

Like BigJim said, cleaning the carbon from the pistons can be very helpful. Mopar sells a very good combustion chamber cleaner, you can get it at your local dealer.

I've had cracked flex plates sound like a knock or heavy ping, too. You can usually isolate one by loading the engine against the brake with the trans in gear and listening for the noise at the bellhousing area.

Exhaust leaks from a cracked manifold are usually more pronounced when the engine is cold and under a load at low rpm.

Hope this helps!