Grinding sound from rear end area when driving

[Crawdad]

I have driven my bronco about 500 miles since I've had it. I have noticed that there are no leaks coming from the drums to suggest my seals are bad, but does that mean the bearings are still good? If that isn't the problem, think it could be from the pinion area, say the pinion bearing? Again no leaks are coming from the pinion seal. Heck for all I know the hothead could be low on oil. It's so dang difficult determining exactly where the noise is coming from when ur in the drivers seat! Any help on ruling potential problems out is most appreciative.
Matt