I have a 1991 explorer that I bought new...still running well, but it has a 4-wheel drive issue. The shift motor engages the transfer case and the front axles seem to turn with the hubs locked in, but I have no power to the front wheels. There has been a clicking noise up frontin the past when the 4-wheel was ehgaged under pressure, but now I have nothing. Is ther a keeper key in the differential ( or something like that) that could be broken? What is between the transfer case and the hubs tht could be a problem? Thanks!