Re: Whirring from rear end at 60mph
Sounds like it could be the rear diff pinion bearing. Due to the pinions faster rotation, it is often described as a whirring noise (as opposed to the carrier bearings which are described as more of a rumble). If your lucky, it could just be the preload is not correct (has any work been done on the rear, like a pinion seal replaced?), though usually that results in a howl when off the gas, not when accelerating.
Have you checked the diff fluid level to ensure it's alright? Noise at a set speed can point to improper lubrication.
If you know what you are looking for, you can remove the cover and give things a look, if not, you'll likely have to take it somewhere for them to give it a once over.