does anyone notice a whistle from their rear diff?

I don't think there is anything wrong, my '13 and now my '14 do the exact same thing. If you're on the gas it whistles, let off and it goes away. It's very faint, but I notice it.

2014 TRD Sport with E posi.

 

Sounds like diff. whine due to bad bearing preload. My 2012 did that - only at light acceleration - OK at coasting and heavy acceleration. They changed the guts in the pumpkin.
I believe I read a TSB out on this for at least 2012. If this sounds like the issue check for tsbs on your year - if not listed, copy the 2012 TSB and take a copy to the dealer and Then take the dealer/Tech on a test ride and show him the TSB and tell him it might be related to the TSB - but regardless - tell him to fix it

 

It's probably a lemon. I'd take it back, that's your best bet right there.

 

Check the oil level. Mine was nearly empty from the factory. Good thing I checked it after I bought it new.

 

LOL .... once is enough, hopefully never again!