2003 Ford Expedition Axle Nut snapped off flush

I have a strange clicking noise coming out of the front wheel on my Expedition. Mechanic says theres lots of play between the hub and spline on both sides.Also the small nut located behind the little dustcap is snapped off.(passengers side)He says hes never seen this before. Id like to know what may have caused this before I spend money to get it fixed. He wants to do both axles and hubs. Your thoughts? Thanks