If you aren't noticing a vibration so much, yeah, I'd suggest you change the fluids in the differentials and transfer case(TC)/VCU. If your Explorer has 4WD it will have a transfer case, but if it's AWD then it will have a Viscous Coupling Unit (VCU).
There are also several posts on this forum related to rear wheel bearings and pinions that can cause a hum at highway speeds. You might search the forum for more info on those.
The binding that you describe does sound like it could be an issue with an outer CV joint, but as the differential and TC/VCU oils are maintenance items anyway, you might change them to see if that has an effect on the hum. If it does, the issue may be more likely to be a bearing or pinion issue than a CV joint.
-Rod