there are two speed sensors , one is $107 and the other no longer available , there are two speed sensors so the diff doesn't engage if the speed is too much to cause damage to the diff , there is a $600 actuator that engages the 4x4 , there is a $180 clutch control unit , and lots of wires in between , look at the manual and count the flashing light code
I would start with figuring out whta code you are throwing , then unplugging every connector in the 4x4 actuator , speed sensor , control unit circuit , check for corrosion , clean and dieletric grease them to start , make sure you have a fully charged battery
if you find you need any of the front diff parts , like the sensors or actuator , I'd sell you a complete diff for $300 plus shipping , comes with 2 sensors and the actuator and the complete front diff and drive shaft , which the front diff is also no longer available as a assembly