[jonmon6691]

Anyone have an idea what could cause a whine with these properties:

• Only audible when coasting down above 60mph in gear
• Not audible while constant speed or accelerating
• Goes away when coasting in neutral, but also there's no load on the drivetrain in neutral
• Pitch/volume doesn't change when shifting to any different gear
• Only audible after about 20min of driving, needs to "warm up"

The noise itself definitely sounds like a bearing or possibly gear teeth, kind of like when you go really fast in reverse on a tranny with straight cut gears.

The fact that changing gears doesn't affect the noise makes me think it can't be engine, throw out bearing (or whatever is between engine/trans), clutch, or trans gears. But I'm not entirely sure what's downstream that could be a culprit.

[pungo]

Mechanics stethoscope while on a lift, that's how I found mine, which was a wheel bearing, though I didn't have the same symptoms as yours.

Your symptoms point to a diff going out, $$$
Have you changed the fluid recently?
If not, check the level, change it and closely inspect the oil for metal using a magnet.

[Wesasxm]

Disconnect the driveshaft do a road test

[Ivan67]

Does the noise increase/decrease while making sharp turns?
I did notice that when I turned left, but ended up replacing both rears
The left one was the culprit.
Good luck,
Ivan.