HELP: Grinding, ROARING rotating sound coming from rear

[Wayne's World]

Was on a high speed run on an open stretch of highway. Got up to about 105. I had to brake fairly heavily at one point because of traffic but nothing serious or emergency type braking. Since that point, it was about 10 mins until I got to my exit. Got off the highway and while slowing down on the off ramp, the car just jerked and slowed down big time as it down shifted. Since then there's a really LOUD rotational noise coming from the rear.

Symptoms include:
My entire neighborhood can hear it
Is speed sensitive
More prominent and apparent throughout the cabin at speeds below 20mph
Above 20mph, it fades a bit
Taking my foot off the gas at speed takes the noise out of the cabin but as soon as the car/suspension settles, it returns to the cabin.
Putting the car in neutral doesn't change the presence of the sound. It goes away slightly but only because i'm taking my foot off the gas. [same sound level]
Engaging drive again, the sound immediately returns to the cabin.
Sounds also like i've got a flat or running on off road tires.
Strong smell of gear oil

Jacked up the rear end and I tried the wheel bearing test of holding the wheel at 12 & 6 and 9 & 3 to move it but there's no movement. Rotating the wheel I hear a scraping noise almost like the pad is contacting the rotor surface. I hear this on both sides.

I've also drained the differential after my short (1mile) drive this morning and the fluid was black. When i did the drain and fill on the weekend prior, i did it cold. For the service I used Valvoline SynPower [full synth] 75w-90. It took approx. 3/4 quart. Today however only 1/4 quart came out..... I stopped the collection when it became a trickle instead of a steady stream.

I've also recently done a brake job with new pads all around, new rotors on the front only and fresh brake fluid.

Some vids:
Noise from inside the cabin

Diff Fluid

Right wheel turning by hand

Left wheel turning by hand

Right side of differential

Front of differential

FINAL UPDATE: This turned out to be my rear differential. It was replaced with a used unit and the car is functioning properly again.