[madmaxx]

In the petrochemical industry we employ about 4 techs per sight. Every gearbox, turnbine, mixer, motor is monitored. If you identify the bearings in your transmission and rear end, gears and number of gears per gear. Remove the shift boot on your transmission so the tech can place his probe on your transmission while you are driving he will tell you exactly what component is generating the noise in under 5 minutes. To test the differential he may be able to rig a probe since it is not real accesible from inside the car at 55 mph.

Option two is to let everyone guess what the problem is and only by the grace of God and alot of luck will you find it without spending alot of money.