I am thinking it could also be our weak tranny mount. I will be changing this out and looking to change out all bushings dealing with the rear diff.
I can produce this sound in all gears. Under heavy acceleration it is very loud and sounds like rubber stretching and grunting . It is a little scarry going down the track listening to this noise which cant be good.
I may be wrong and I'm sure someone will step in if I am, but when you lower your car and/or stiffen up the springs you reduce the squat and the torque that goes to creating this squat now looks for other weak spots to bend and twist. My guess is this torque is having lots of fun with bushings and mounts on our driveline. This is just a guess, but sure does sound good .