Drivetrain whining going downhill on freeway when I release foot from gas pedal on 2003 GMC Sierra 2500 HD

I've recently had a rebuilt tranny/transfer case/torque converter installed on my truck. When I've been driving for a while on the highway, and I let my foot off the gas while on a decline, I hear this really loud scratching, whining noise that last for about 10 seconds and then stops. It feel as if it is falling out of gear or maybe losing pressure of some kind and I cant diagnose it because it only happens at freeway speeds. Any ideas whats wrong?

Asked by for the 2003 GMC Sierra 2500 HD
sounds more like a differential issue than transfer case or trans. the pinion bearings give this type of noise . have it checked to confirm