problem w/ front differential in '95 XLT 4x4

Does anyone know a good way to isolate/confirm failure of one of the bearings in the front differential of a '95 Expl XLT 4x4? The vehicle has 110K miles. The mechanic we use for routine work believes the problem (low frequency sound) is in the differential. The transmission shop we went to thinks its tire noise. Since we just got new top of line tires, I went to the tire shop and their alignment mechanic also indicated he thinks it's the front differential. We went so far as to have a different set of tires put on temporarily just to check, and the sound was still there. I'd appreciate any ideas or suggestions on how to diagnose the root problem. Thanks.