What is the whining noise from the back of 2006 Toyota RAV4?

From the rear it could be the differential, however those front differentials are notorious for making noise. That whining noise derives from the viscous coupler, and is normal. Bring it to a dealer, not some crappy backyard mechanic. And have a certified Toyota TECHNICIAN inspect the front an rear differential, transmission, and wheel bearings (but those typically hum when bad), I almost gurantee you just hear the viscous coupler noise.