is there not a connector a few inches up on that? if you unscrew that switch you'll have transmission fluid coming out.. and DONT START the truck with it out and press or reverse or you would SPRAY fluid everywhere..
if you pulled it out, and key on but not running.. and are able to measure 12 volts across the 2 wires of that switch (at its plugin) then you have a short to ground downstream of the switch.
I believe that switch is on the positive side.. I dont have the diagrams handy for that but if I remember right its on the + side..