Welcome to GC, rodtoney. First, check your fuses, I believe there is one for the 4X4 function.
for a sploded view of the diff.
Get hold of a wiring diagram (I have one at home but no access to it right now) and trace down the wires to the diff servo motor #24. Verify that it gets the proper voltage at the motor connector when you engage/disengage the 4X4 switch. If it does not, your prob is in the switch or the wiring (or fuse).
If you find the proper voltages at the diff servo motor connector but it's still not working (and the connector is not all corroded or damaged), drain the diff oil and remove the servo motor from the diff. Engage & disengage the 4X4 mode and see if your servo motor turns one way and then the other when you engage/disengage. If not, the motor is bad.
Even if the motor is bad, go ahead and manually check the diff 4X4 engage function: you should be able to see the linear gear #27 that engages with the gear from the servo motor. Sliding this linear gear back and forth engages/disengages 4X4. Try to slide it in and out by hand (you will probably need to turn one or the other of the front wheels to get this to happen so having the front end jacked up can really help). If you can't get it to engage, you probably have a mechanical issue in the diff...even if the motor is gone.
Don't forget to refill with proper gear lube after reassembly.