Chances are the issue is electrical. Step 1, I would check the switch (wires/connections) and see if they are putting out 12 volt. These switches do fail, so hopefully its a real simple fix for you.
Ditto on the above. There is a connector about 12" from the front diff'. Check for power there. If you have power, check where the wires enter the diff'.....I've seen them get chaffed there.
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