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Is your actuator the 2 wire or 4 wire actuator? It is in the front axle tube, passengers side. It threads out. YOu can test by taking it out and leaving the actuator plugged in, then tunring the key on and putting in 4wd.
Ok, the actuator you have is a 2 wire thermal actuator. IT extends when the wax in there heats up.
First see if the 4wd or tcase fuse is bad. OFten when the actuator failes, it pops that fuse. Otherwise if you have a test light, put it in 4wd and test for power on the wires to the actautor.