1996 4WD clicking and won't engage
 

I have a 1996 isusu rodeo 3.2 v6 LS edition. When i purchased it a year ago I had no problem engaging into 4WD moving or stopped. But about a year ago, it started taking longer to engage, minutes even. It didn't mater if i was stopped or moving (forward of reverse) eventually it just stopped engauging all together. and recently there is a slow slicking noise ( metal on metal every.5 seconds at 20mph) that clicks faster with speed. Its coming from the front of the car. When I try to engage the 4WD it turns into a low decently loud rumble. The 4WD light won't even turn on and blink anymore. Any ideas on where to start?