Whining noise from rear end 99 Explorer

[ventura77]

Hi there,
My 99 Explorer Limited has 64k miles on it. Within the last few thousand I have noticed a whining noise coming from the back end. It does NOT seem to mater whether I am on the gas or coasting, turning, whatever it is always there. It seems loudest and most noticable at highway speeds of 55-65. Also when coasting to a stop, but that may be only because the engine is at idle so I can hear more.
I have actually sat in the cargo compartment while my wife drives to try and detect which area of the truck it's coming from. It's really hard to tell!
Ford recently replaced the rear pinion seal and axle seals. I am not certain whether it started immediately after that or not.
I am thinking it's some sort of bearing, although rear diff and tcase also come to mind. Tires also come to mind, but it does not really sound like that type of noise. Anyone else have this problem? Any ideas or things I can do to test?
Thanks!