It is unusual for a 235 with 76000 miles on it to have rod bearing problems. I suppose the speedo could have been changed at one time or had a broken speedo cable.
You can drive the truck with one plug wire off at a time to test. I would just make sure the wire is grounded so the spark does not jump somewhere you do not want it to.
I also wonder if maybe you are hearing the engine ping. Does the "rattle" change depending on the load you put on an engine. A rod knock usually is heard just as you let up on the gas. Pinging is heard while stepping down on the gas.
If you do pull the pan than yes that is the time to check everything and change the rear seal.
