a posi loc is a cable actuator to engage your front axle.
your YJ has a 'two piece' passengerside axleshaft. when you pop it into 4wd there is a collar that slides over the two shafts (thats what she said) which makes the axle a solid one piece. to move this collar there is a vacuum powered fork that slides the collar. it is very common for this actuator to stop working; and it doesn't engage the collar/axleshaft; in turn, this will not make your front end work; because the pass. wheel cannot be driven, it'll always spin and not be real 4x4. (i'm having trouble explaining it)
the posi loc is a cable actuated lever type deal where you manually move the shift fork to engage 4x4.
OR, you toss a couple washers on the fork and leave it engaged all the time.........................
google it, you'll get some pics