You might want to pop the driveshaft off the rearend to isolate the noise. At least you'll know if it's at the rearend or not. If the rear is quiet, then see if you can make the noise at the trans by turning the shaft by hand. If it does, slide the yoke out just a little bit and see if it still makes noise, just to make sure no part of the driveshaft is bottoming out.