Basically four different options here. 3 sp "Brownie", 4 sp "Brownie", GV OD Unit or US Gears OD Unit.
Truck is "potentially" 1992 Chevy Kodiak LoPro, 5.9 Cummins (mild), AT545, 60" sleeper, 9' flatbed, 245/70R19.5 tires, weight ~10,000#'s, pulling 10,000# gooseneck horse trailer or...