I think it should work. I looked up 85 5.0L front wheel bearing dimensions and 2000 Ranger wheel bearing dimensions (on rockauto). It looks like the OD's of the 85 Mustang and 2000 Ranger are the same (both inner and outer bearings). The ID's are different, so I think you could use your Mustang bearings in the Ranger rotor. I have not tried it, but if the dimensions I found are correct, it should work.
Pick up a used rotor at the wreckers and try.