Easiest check would be to unplug the 4wd switch, you might have a 4wd switch that is stuck closed sending 12v to the hillard clutch. Worse case scenario is you clutch failed internally
try driving in reverse sometimes they will come out. but it is usually on its last legs at this point. they need to be maintained on at least a yearly basis if not twice a year(oil changes).
I change my front diff oil every oil change, my brothers was doing the same thing and we flushed all the slime out of the front diff and all seems to be good now.