You say you here sounds from the rear axle, more often than not the RDU has a problem.
I don't know if you have the Ford powertrain warranty of 5 years 60,000 miles where you are, unless you are past that mileage, the powertrain warranty would cover the RDU. I also don't know if TSB's apply in your location, I would ask a Ford dealer to see what they can do for you if anything.