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Are you positive that the noise is coming from the differential? Feathered/chopped tires can make a noise similar to a bearing, and rear hub bearings can also make noise. You should be able to rule out tires with a visual inspection. Take note of whether the noise changes or comes and goes when turning, this loads and unloads the hub bearings. If this does change the noise then you have a hub issue.
If the noise is coming from the differential and it's a roaring rather than a gear whine then it will either be a failed pinion bearing or carrier side bearing. Either of the two are about as likely as the other. When would need to be done is disassemble the differential for inspection and go from there. If you aren't hearing any whining then the ring and pinion gears should be ok, if any of the bearings have failed then standard procedure would be to replace all pinion and carrier bearings.
Thanks for your reply. I thought about the tires and had them inspected. There were no issues with either belt or tread wear. The noise is present only inn the highway between 40 and 60 mph and is very consistent. You have confirmed the likelihood of differential damage. I will proceed on that basis when I seek repairs. Thank you.