Drained the diff, removed it, cleaned it, popped out the output shafts (carefully) with a tire iron, removed the old seals by tapping them (carefully) with a screw driver (normal seal puller would not get them out), put the new seals in and seated them with a seal install tool, took off the old cover, put on a M Coupe diff cover, put the output shafts back in, re-installed the diff, filled the diff with RedLine 75/90 (RedLine with LS additives).
First test drive I could hear the whine loud and clear. Diff is not leaking anywhere, double checked the fill level. I had the rear interior panels off and back seat out (replaced subframe mounts and trailing arm bushings also) and the noise is very loud.
I had someone ride in the back seat a few days later and they could hear the sound from both sides. Then they drove along side of me and could hear the noise from both sides. Sounds like an electric motor.
Before replacing the seals I could not hear any noise. According to my records the diff has been filled with RedLine every ~50K miles for all of its 200K mile life. Any ideas?