Would it hurt to run a heavy duty diesel coolant in my '84 F-150? Or in my wife's Dodge Caravan for that matter. I already run a 15w-40 diesel oil in them.

 

Some of the diesel coolants do not have silicate in them. That protects the aluminum parts in your cooling system, which can be the radiator and the replacement heater cores are usually aluminum.

If it's low silicate coolant, make sure it has some other stuff in it to protect the aluminum.