The noise on decel is "something" changing the pinion gear angle, OR the pinion gear is set at the incorrect depth. As mentioned, check the pinion yoke nut. If it's tight, chances are good the pinion gear is set incorrectly. As for the clunking, IF it's a limited slip type (you didn't specify, so we don't know) and you did not add friction modifier, that could be your problem. Clutch type or cone type both need that to help keep operation smooth and quiet.
