Before I did anaything else, I'd find some place to put the rig up on a hoist, put the t-case in Neutral, and have someone spin each rear tire in turn by hand, while you get your ear close to different areas to find out where the noise is coming from. First off, if it's a suspension problem, you shouldn't get any noise with this method. If you do get noise, it would be easy to isolate it to either the driveshaft U-joint, the internals of the differential, or something in the outer hubs. If it's inside the differential, I'd remove the diff cover if you have time and again have someone spin tires by hand while you isolate the bad area.