I have a 99 ford ranger 4x4 with 4.0. Why does 4x4 work sumtime an not sumetimes

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The last serveral times iv tried the 4x4 it hasnt worked. When i switch it to 4 low it doesnt even change to low ratio it jus stays in high ratio an i cant here nothing clicking in the transfer case. The front drive shaft does turn when spinning when in four wheel drive so somethings not letting it click in. It worked three days ago jus fine when test driving it but now cant get it to. Thinking it mite be the 4x4 switch inside the truck. If so how can i hot wire tht to c if thts the problem? What wire would i cut an tire togather? If anyone can help i would appreciate it. Any advise is sure to be better then what iv gotton.