battery or engine?

Hello, I don't think it's the battery or engine, but there are too many possibilities to speculate without scanning the truck. A diagnostic fee sounds bad, but the first part you replace in error would have paid for it... even a battery. Are you losing electrical power or is the engine just turning off? If you are losing electrical power, but it comes back, you might just have a bad battery terminal to battery connection, corrosion, or you could have wear on the positive battery cable. If the engine is shutting off it could be anything from the key or ignition switch all the way to a cam position sensor or SAM unit. You really do need it diagnosed if nothing is obviously the matter.