A whining/grinding noise when you're decelerating could be caused by worn pinion bearings in your rear end. The wear in the bearings allows the pinion to move in towards the the Ring. You wouldn't get the same noise on drive as the pinion is pushed back towards the front of your truck while accelerating or on power.
If you have an open drive, check for backwards and forwards movement on the rear end yoke; there shouldn't be any at all. If your truck has an enclosed driveline it isn't easy to check for movement without taking the driveline apart.
