How often?
Senior Member
Joined: Nov 2013
Posts: 4,059
Likes: 396
From: Kansas
Change the trans fluid YES but Ford does not recommend that you change the differential unless you have reason to believe that water has gotten into it. A lot of people think you should change it regardless of what Ford says and it that is what you want to do; go for it.
Senior Member
Joined: Jan 2015
Posts: 231
Likes: 34
If I were to buy a used truck and didn't know if the PO had taken good care of it, I would change it just in case they ran through deep water. Plus you can look at the gear and make sure everything is good. I realize Ford doesn't recommend it, but it's good preventative maintenance in my book.