Loud singing/whine from rear just under 40mph. Diff? Wheel bearings?
 

So I have this loud singing/whine from rear just under 40mph.

Diff?
Wheel bearings?
Or just my Toyo R888s?

It's fairly low pitch, if I had to guess around 800Hz, a bit like this .WAV at the end: http://www.e2s.com/45%20tones/tone42.wav

Around 37-38mph (60kmh) it's the loudest.

It is totally unrelated to throttle/clutch position and gear selection. I haven't tried cornering yet.

Take a test drive and weave left and right. If the sound changes with wheel loading it is probably a bearing. You can also check the bearings by jacking the car up and checking for play. You should feel very little or no play in the wheels.

Did a bit of slalom and it doesnt seem to change the noise...
NEED MORE INPUT!

Jack the rear, run it in gear... tire noise will go away. Can leave in gear and listen to bearings and diff.

Goodluck

Or swap tires out.