94 Silverado - Rear End Noise

I have a 94 Chevy Silverado, 6.5L diesel, with 200k on it. Recently I've been noticing that whenever I accelerate a whining noise seems to be coming from the rear end. I changed the differential oil but this had no effect. I was wondering if this could be caused by a worn ring and pinion gear in the differential. Also, my engine is reving unnormally high, could this be related to the rear end. Any suggestions would be greatly appreciated. Thanks.