code: -51
from my recollection
this can be caused by a couple of things
-battery cables too long
-Tag lines without filters are being used (doubt this is it but the manual lists this as a cause)
- High peak current in motor, possible shorted field windings or shorted armature winding.

even though these troubleshooting suggestions are just that and you most likly will not find anything wrong there, i do see it shows the current sensor in the troubleshooting diagram. Have you checked this? Check the motor itself for grounds to the frame from positive and negative both. It could be picking up residual induction through the motors and causing the "shorted" effect, would explain why it blew the control fuse on keyswitch.

Try pulling the battery out of the truck, see if the code goes away(case bleed through can cause issues)
blow out the motors, carbon dust can cause havok as well if the insulation in the motors are breaking down.

so you have some reaction on the system with key off? AND with it ON as well? just flipflops back and forth?
what happens when you remove the key from the equasion? plug in the battery?

almost sounding to me like you have a control wire shorted/melted against something, this would explain the blown fuse and the strange readings your getting on the keyswitch.

Just some things to ponder while running around in circles ;o)