when i drive my truck though deep water, it always gets water stuck in the rear differential. this is a real problem when i have to back a boat into the water or somthing. why does this happen?

 

Because there is a air vent at the top of the axle tube housing.

75w-140 weight oil also tends to hold onto the water compared to lighter weight oils. This is a very common problem for Fords.

I would just change your oil regularly at 15kmi and you shouldn't have much to worry about unless you park your truck in the water overnight to load your boat??..lol

 

When I used to tournament bass fish there used to be some spots with pretty shoddy ramps, make it necessary for me to have to back the truck into deeper water than I would have liked.
Always had a little milky look to the diff fluid in the rear when I changed it, but as 'dog advised, I changed it at 15k and never had any noticeable problems...

A little bit of water and regular changes, no problem... Any significant water = change right away...

 

Have you inspected your axle seals? I've seen a lot of trucks (all brands) that have oil around the axle in some location or another. Maybe you could modify the breather tube under the bed, so that it reaches farther forward, or higher than the current place? If I were you, I'd be curious to exactly where it gets in; if its not every little seal location. I change my gear oil on a regular basis, which is why I bought my PML cover with a drain plug.