First I would eliminate the front. Take a test ride on a smooth road. Lock the hubs in, if the noise goes away or changes greatly then it is the spindle beairng in the front hub. If the doesn't do anything then next jack up one front side at a time, remove the wheel and brake caliper. That should let it roll without any brake drag. Put the rotor and wheel back on thighten it up with a few lug nuts. Roll it it should roll smoothly with no grinding or noise. Take the wheel and pull in in opposite directions at 0 and 180 degrees, top and bottom. There should be no discernable movement in the bearing. Check both sides. If it is not there then it is likely the U-jonts in the drive shaft, drive shaft hanger bearing or a rear differential part. To diagnose rear differential problems is tricky but here is what they sound like, and sometimes it SEEMS like the sound is coming from the front.
1. A whirring or gringding noise at all seeds is most likely a bad pinion bearing or one with loose preload.
2. Howl or whine during acceleration is usually caused by improper gear set up.
3. Rumbling or whirring at speed over 20 mph or so, seems to go away at highway speeds, is usually differential carrier bearings.
I went through a time with my truck when I changed hub both front assemblies chasing a problem, wasn't it. It was the pinion and carrier bearings in the rear differential. Let the dealer (mistake) rebuild it and they messed it up, lassted 18K. When it did it a second time the sound again seemed like it was coming from the front, might just be the internal accoustics of a crew cab. Anyway, this time I went into the rear differential myself, very bad drivers side carrier bearing, pitted pinion bearing races from the shavings, broken differential shim. I think that they broke the shim getting the preload and either didn't know it or said the heck with it. I am finsihing up the rebuild today.