More description would help, but here are a few things I'd look into first. I'd get the rear wheels up and check the hubs/bearings. One good way is to grab the tire top and bottom and try to rock it back and forth. There should be ZERO play. Also, you can rotate the wheels and see if you hear anything from the hub.
You didn't say if there's also vibration, but if so, suspect the U-joints. Also, each of the halfshafts has two joints, and these make noise when they get worn. Just to be sure, I'd check for oil leaks around the differential (especially the seals where the halfshafts go in) and I'd also check the oil level in the differential. Should be right at the hole when you remove the fill plug. Finally, make sure nothing looks strange with any of the tires. A tire that's defective inside can make a lot of droning noise. (But no grinding noise)