Did he replace the pinion bearing with the gears? If not, I would look at that first. In the description section for Ford Racing gears on the American Muscle Website is the following note:
Assuming that the above is not the problem:
Is the noise all the time or when coasting with your foot of the gas? If is when coasting it is bad pinion bearing. Generally a steady gear whine is due to faulty installation such as incorrectly adjusting the back lash or defective/damaged gears. When you say that he has redone the gears four times do you mean that he reinstalled the same ring and pinion 4 times or that he installed 4 different gear sets? If he installed different gear sets then you can pretty much eliminate defective parts. If he is reinstalling the same set then it is possible that the 1st installation is done incorrectly and they were damaged as a result. There are also the ridiculously stupid possibilities that also have to be eliminated, such as forgetting to refill the diff, or some how reinstalling the
panhard rod so that is touching the diff cover (if that is even possible)
Well good luck and let us know how it turns out.