low sounding singing noise in rear end
Sounds like it might be low on oil.
2WD or 4WD? Has it ever been in an accident? Does the volume or pitch change as the speed increases?
Could it be possible tire whine? Does it do it all the time or only on certain road surfaces and not others?
Yes, it could be tire whine, but I think it behooves the OP to check the level of lubricant in his rear differential before he even considers the tires.