Here is a discription of how the front diff. locks into 4x4:
The shift motor receives a vacuum signal from the switch mounted on the transfer case when the vehicle operator wants to switch from two wheel drive mode to four wheel drive mode, or vice versa. When this signal is received, the shift motor begins to move the shift fork and collar within the axle housing. In the four wheel drive mode, the shift collar connects the axle intermediate shaft to the axle shaft to supply engine power to both front wheels. In two wheel drive mode, the shift collar is disengaged from the intermediate shaft and the intermediate shaft is allowed to free-spin. When the two shafts are disengaged, the load on the engine is reduced, thereby providing better fuel economy and road handling.
Here is a picture of the vacuum actuator and its location.
And here is a picture of the vaccuum lines from the transfer case to front diff.
I would recommen dwith starting with checking vaccuum at the proper locations. LEt me know what you find and I will help you further.
I havent found a vehicle to varifiy it on yet, have you found the issue yet? Let know.