For me, the sound seems to be related directly to speed.
It doesn't change whether I'm pressing the gas or coasting - So probably not the gear mesh?
I took the brakes and rotors off and spun it up - same noise, so not those.
It doesn't change when I do tight s-turns in a parking lot - So probably not axle bearings?
What's that leave as possible suspects? Pinion bearings, carrier bearings and the universal on the driveshaft?
I have no idea why they didn't just replace both carrier bearings and both pinion bearings along with the bearing races the first time they opened it up. That's gotta be cheaper than opening it up multiple times trying to hunt it down.
Or, I suppose they could just pretend they fixed it. Seriously - I started a case with Ford of Canada and when my agent checked in with the dealership, he was told that they took care of it. I guess throwing your hands up in the air with a bit of a shrug counts as 'taking car of it'? Unbelievable.