1998 GMC C1500 Rear differential noise

Just bought a 98 half ton ext. Cab from friend, Know history of truck. Has been making abnormal diff. Noise for 7 years. Recently replaced outer and inner axle bearings and lube, same results. Pinion bearing seems ok. Gears in diff. Looked good also. When accelerating or slowing down in drive have rotating noise similar to a slight grinding or rubbing noise loud enough to be very annoying and making me curios as to the life expectancy of the rear diff. Noise will quiet somewhat when coasting and gas pedal is pressed slightly. Any help you can offer would be greatly appreciated! Thanks, Ray