with the type of shifter control on this truck, first i would verify that you are getting a good signal from the shift control to the solenoid circuit. Those shifters are known to wear out internally, the contact fingers wear off or break causing no signal to the solenoid.

assuming you werent bulldozing with the truck
check the fluid, make sure its not burnt, burnt fluid usually smells really bad and is dark.
if you ARE getting a good signal from the directional control to the solenoids in both directions and the ground is good

you can also try swapping the solenoids and see if the problem moves to the other direction, if it does then you know you have a bad solenoid.

just a couple things to ponder

good luck!