I don't understand- this is the replacement differential that is whining?
A whine from the rear is, yes, probably the diff.
Wheel bearings make a moaning or grumbling sound (gotta love descriptions of sound), and would not let up with your foot off the accelerator.
Axles make a nasty thumping (ask me how I know).
If you got a replacement, chances are the fluid level is good. Adding more will not help. Some folks like to put a thicker fluid in there (straight 90) to reduce the noise some. But it probably won't get rid of the whine altogether. It's a worn differential when it starts whining.

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