Hy retcon, did a reputable shop rebuild the differential, or did a backyarder do it? A lot of noise can be lessened or eliminated by the proper setup of the contact pattern between the ring and pinion gears and setting the proper backlash. If your original gears have a lot of miles on them they probably have wear patterns in them, when setting up used gears you have to be careful not to set them highpoint to highpoint, sometimes a little more backlash can help avoid this. If a shop did the work, then I would take it back to them and ask what they think is causing the rear axle to howl. It may be the case that a new ring and pinion gear are required, this may be the cheapest way to resolve the problem, you've already shelled out for new bearings which should be re-useable. The other alternative would be a whole rear axle swap using a wrecking yard rear axle, hope that helps.
