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You're having the problem that many of us with Gen 1 explorers have: the OTF 4x4 system was, ahem, not robust. The click that you're hearing in the back is the 4x4 control module, behind the panel where the jack is stored. The problem is either that that module is shot, or the shift motor on the transfer case has seized up. In either case, a search of this forum will turn up lots of information.
Good luck... I still haven't fixed mine.
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