Contrary to what a previous poster said, Ford seals IMO are not that great. I've had an axle seal and a pinion seal fail on mine with less than 100K miles. I've had 3 X's and there always seems to be one seal that leaks quickly.
As far as water being in the differential, I think that IS bull crap, and just a scare tactic. You 'might' have a leaky seal, and that is easy enough to tell for yourself by looking at the axle seal in question and there will be obvious wetness compared to the othere 3 wheel areas which would be dry.
Assuming that 1: You DO have a leaking seal, and 2: That you DO want to replace it. 3: It's not necessary to do a complete drain and refill of the differential, unless you want to and it should only take a couple of quarts maximum as previously mentioned. The dealers kill us with fluid costs. Hardly any fluid willl leak out during the replacement.
I recently got an estimate for my pinoin seal replacement and the dealer told me that he had calculated almost $100 for 4 qts of differential fluid. Newsflash: It only takes 2 qts!!! I told him no thank you on the fluid and I'll top it off myself if it needs it. Total pinion seal replacement cost including a NEW pinion yoke was less than $200.
Last but not least, you can run almost forever with a minor leaking axle seal as long as you check the fluid level once in a while. My wife went for a couple of years with a leaky seal before finally replacing it. Probably only lost less than 1 pt of fluid in the 2 years, and got free rustproofing from the oil creep under the left front of her vehicle.
