heres an easy test to see where the problem lies. start it up, put it in 4x4 and jack up a front wheel and give it a spin. does it spin freely? if it spins freely look to see if the front driveshaft spins with the wheel or if it does nothing. if the driveshaft doesn't spin then the problem is in the front axle. remove the battery tray to gain access to the vacuum actuator and make sure it is pulling the cable all the way. 2 common problems i have found on these trucks is either the actuator takes a dump, or the cable gets stiff and either won't engage, or disengage the diff.
"I don't have pet peeves, i have major psychotic fucking hatreds" -George Carlin