First of all, take it to a reputable Ford servicing dealer - yes they aren't all reputable. I'd get 3 different dealer's ideas first without suggesting anything to them - let them work it out!!
If you have no more luck, ask them to machine the discs for you - it is the pads vibrating against the discs slightly - if the machining doesn't fix the problem, swap over to different pad material. Apparently Bendix make a "4WD" brake pad.
Have a look at www.bendix.com.au
and also www.discpads.com.au
for an idea of pricing.