Mine whines, and i know it's the pinion, i regeared it and didn't set it up totally correct, as it is just a temporary thing until it gets rebuilt with the locker later down the road, hopefully in the next two months. But yes thats what it possibly is, depending on the "sound" mine is a higher pitched,faster sounding whine, mine is also caused by the pinion nut not being torqued to spec, with my tools it is darn near impossible to get the torque to spec, and also the fact,,,and yes i know,,,that i reused the original crush sleeve.
Like i said, it's temporary till the rebuild.
If your going to tear it apart to replace one set of bearings, i would just do the others as well, to make sure the job is done right, unlike mine.