Put the rear on jackstands and let it idle in drive. See if you can isolate it that way.
Hate to say it, but the frequency makes it seem like the driveshaft to me. Is anything near it or touching it?
Your new rear end, do you know if it has taper or roller bearings? If they're taper, did you pack them with grease? Does the noise go away when you drag the brakes? If also check the dust cover on the Bellhousing. Maybe it's hitting the converter? Or is just loose and rattling?
So many different possibilities.
