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.

Pinion bearings in the diff usually cause what you describe, the rpms is another issue.