whining sound in rear end

If this is still not repaired ford has a ring and pinion kit for this usually bearing and shim wear inside rear diff causes this not a cheap repair just make sure they clean out the old metal from the bearing failure so it doesnt happen again

It is not a sealed unit, & the fluid level should be checked every service of the vehicle.

If it's making noise then the damage has been done and you may need to have the rear end rebuilt. AKH is right, get the fluid checked at every service.