I'd remove the actuator from the diff and see if you can move the shift fork by hand to get it to engage. If that works, then try to get the actuator to actuate by pressing the button on the dash with it in 4LO. If the actuator turns, it may just have to be reinstalled properly. (They kinda have to be timed...) Do a search and see if someone has the instructions scanned from the factory service manual. I might be able to photocopy and scan the stuff from a book at work if you need....
Ryan
