Well the rear diff had crossed my mind but the unit I'm running I bought off a member on here that only had 500 miles since a rebuild (by dr diff actually I believe). I then had the 3.55 gears swapped out for 3.91s (used) and had a new pinion bearing installed. The guy that did the install said everything looked fine inside.

To eliminate the rear end, I should be able to put the trans in neutral and if I still hear the whine when I give it some gas, that would indicate its probably transmission related correct? If it doesn't whine then its probably a rear end problem correct?