My 79 Macho has the 8 3/8" rear. It has a 360, and 3.21 gears. Seems weird that a 4x4 has the smaller differential with such high gears, but that's how it came from factory. I think Randy's Ring & Pinion Web site has picture references to what would be in your truck.
You're on the right track, clean the T-case as good as you can, and keep an eye on it. The leak point(s) will show up.