Hi! Seems to be the brushes on your shift motor are stuck do to lack of use. Locate the shift motor (looks like a big electrical motor, bolted to transfercase) and with the vehicle in park, and the switch on 2wd high, tap the starter with a hammer several times until it shifts. If you get it to shift, constantly switch the motor from 2wd to 4wd Low (remember vehicle must be in neutral with foot on the brake to shift out of 4wdl and into This should free up the motor, and shift it atleast once a week to prevent future failure.
Thanks so much for the question,