I was wondering if anyone could tell me what this plug is for. My guess is a block heater, but seeing as this truck has probably never left SoCal it kinda threw me off.

Any takers?

 

You correct, it is the block heater.
Nick

 

SoCal or South Pole, it doesn't matter. They all have 'em.

Stewart

 

I thought it was just an option. Do all superdutys have them?

 

Not all of them, but most of them. Specially the trucks built for areas that get cold and snow.
Be sure to always inspect the wiring for stiffness (wires getting hard from age and heat) and only plug it in when you need to. From time to time they have been known to catch fire. It doesn't happen often but it is something that is a must know.