For the amount of fluid you would need Ford sells in 1-qt containers so maybe just buy it there. The book for your truck will show you the amount you need also with the syn. lube it shows change interval at over 100,000 miles or more I believe. My truck pulls most the time so I change things earlier and had the rear diff fluid changed already to be safe as the truck is in water on a regular basis and syn. lube water is its biggest enemy I'm told....plus its below freezing here for the next 4-5 months so for me its more of a contamination issue and preventive maintenance

2001 SD 4x4 5.4 auto 3:73