First, what is the fluid change history for the transfer case and differentials. Transfer case needs fluid replaced every 50K miles, diffs every 100K miles.
Can you localize the noise better in the front area? It's possible the splined disconnect is gone. Is the Service 4WD light on in the dash? If not then the actuator motors should be working.
Can you localize the noise better in the front area? It's possible the splined disconnect is gone. Is the Service 4WD light on in the dash? If not then the actuator motors should be working.