That grinding sound from the rear are not wheel bearings; if it is coming from the rear differential, most likely, it is the ring and pinion that is worn out. You have to replace those soon. Otherwise, they are going to get so bad that you will feel power being robbed. You know-you are punching the throttle and you don't feel the wheels respond. The rear differential gets so hot that it glows bright orange, like a ball of flame. Yes, that ring and pinion is definitely ready for a swap. So what causes the rear differential's ring and pinion to fail? Natural wear and tear or perhaps you're too busy that you don't bother to notice oil dripping from the differential. You see, inside that big pumpkin in between your rear wheels is oil-lubricating oil to keep the guts from grinding each other down. Now, if that oil were to leak, then it'll just be a matter of time before something starts complaining. In your case, it's that sound of grinding metal you're hearing. Well, you can't blame anyone else. If it needs replacement, then you've got no choice but to cut the pumpkin loose, break it apart and fix it. If you're not up for a differential rebuild, then have a professional do it. Just make sure you get quality parts. It should take at least a decade before the ring and pinion wears out again. Quality parts are available right here at Car Stuff.