Why do you thnk you need to change the diff fluid after only 16k miles? Did you dunk the diffs in water??
The owners manual says you can can go 100k before changing the fluid, as long as it has NEVER been under water before......
If you want to change the fluid anyway (won't hurt anything), the spec should be in your owners manual. Mine is spec'd for 75w90 for the front and 75w140 SYNTHETIC for the rear. I used AMSOIL that I got from GI Joes. I think it was 2 qts for the front and 3 qts for the rear. You can always just go for the dealers Motorcraft stuff, but they wanted $19.00/qt for the rear end lube!! I got the AMSOIL for $12.00/qt at Joes ($8.00 for the 75w90). You should get a bottle of the "Friction Modifer" from Ford ($5.00) if you have the Limited Slip rear however.
I know there is a specific "spec" for the type of "syn" to use, and when I checked, the AMSOIL had the same spec as the MC stuff..... Sorry, I don't recall the exact figure, but I got it from the parts counter at the Ford dealer.