Been driving my 97 F150 4x4 (with a 4 in lift) through the water up to step bar depth this week due to flooding from Hurricane Ike here.

Figure I should change the diff oil.

What should I use, plain old gear oil or something fancier?

 

Ive got Royal Purple in mine. Good thing about it is you dont have to add friction modifyer, its already in the mix.

 

Theoretically you should be good. The vent line runs all the way up to the frame where the shock crossmember is. That's the only open access to the diff for something to get in. You could pull the refill plug and see if it's over full or mixed with water. I'd bet you're good but doesn't hurt to check real quick.

If you want to change for peace of mind, swap it out with Amsoil 75W140 Extreme Gear. VERY good stuff and odds are your truck will be long gone before you ever need to change it again.