Noise Whine Coming From Rear

Hello everyone. Love this forum, and all the advice.

1999 4Runner SR5, V6, 4WD, stock, automatic, regular differential, +240k miles, I've owned it since new, full synthetic, mostly highway miles, but recently we're building a house, so I've been pulling a trailer here and there. If I'm not driving the 4Runner to work, the family goes to build the house (2 adults, 2 kids, 2 dogs).

We've noticed a whine. It starts when the vehicle starts to move, and eventually gets drowned out at higher highway speeds (+75mph). The pitch of the whine goes up with the speed. The whine occurs regardless if we accelerate, decelerate, or coast in neutral. The whine is there regardless of road surface (smooth or chip seal), and the smoother the road surface, the more you can hear it because there is less competing noise.

I lifted the front, and the wheels are tight, no play and they spin nice with no noise or friction. I then thought it was my worn aging tires, but after replacing all 4 tires, the whine is still there.

I replaced the rear diff fluid (prior was Amsoil full synthetic), but same whine. I went about 40k in-between diff fluid changes. The fluid was darkish, and there were some metal shavings on the plug.

I took the 4Runner to Toyota, and they found the whine/noise coming from the rear, not the front. Toyota wanted ~$640 just to tear down the rear differential to determine what the issue is. I don't have that kind of money right now (i.e. I'm building a house), but will cough up the money if I know what the problem is. I'm not comfortable spending $640 and only then knowing when I'm getting into.

Any ideas? Have any of you run into this?

I appreciate you guys.