I can’t see the crack, is it in the electrode side of the plug, inside the cylinder? That seems to be the case as the 3rd pic is just the inside of the plug. You may have a “hot” spot in that cylinder causing pre-ignition. Do you hear the engine knocking?
Check out the location of the knock sensor. If it is easy to find, make sure the connector is attached and perhaps even change out the sensor.
If the pre-ignition is just in the one cylinder the knock sensor may not be reacting to it. You may need to run a higher octane gas.
Since this is an older truck some of the internal cooling passages maybe plugged. You might want to see if you can really flush out the old coolant and put some chemicals in the block that are designed to clean out the dirt and crud that might have build up inside the block.
In the end replacing a plug every 20K isn’t that big a deal. Carry a spare and the needed wrenches and you can change it in a parking lot if necessary.