Grand Cherokee 4WD problem

Hello Folks,

I have a 2000 Gr. Cherokee that makes a noise which seems to speed up
as I accelorate; something like a branch caught in bicycle spokes.
It's always in 4WD and I do not have the option to change this. I have
a feeling it's in the Transfer Case but would hope that a Jeep with
52,000 miles in excellent condition wouldn't have a serious problem
such as this. Does the "always-on" 4WD create more wear and tear in
the drive shaft?
The noise is constant when the wheels are turning regardless if I'm on
the accelorator or not and seems to come from somewhere in the drive
shaft.

Has anyone heard a noise like this? Any ideas on what this might be?

Thanks!

Micah