I picked a 2wd 8.8 4.10 ratio axle. Apparently they had a 7.5 axle with the 4.10 ratio also. Here are all the different rear axles listed for a 2000 B3000 Mazda.
8.8" ring gear, 10" brakes (4x4), 3.55 ratio
8.8" ring gear, 9" brakes (4x2), 3.55 ratio
8.8" ring gear, 10" brakes (4x4), 3.73 ratio
8.8" ring gear, 10" brakes (4x4), 4.10 ratio
8.8" ring gear, 9" brakes (4x2), 3.73 ratio
8.8" ring gear, 9" brakes (4x2), 4.10 ratio
7.5" ring gear, 10" brakes, 3.73 ratio
7.5" ring gear, 10" brakes, 4.10 ratio
7.5" ring gear, 9" brakes, 3.73 ratio
7.5" ring gear, 9" brakes, 4.10 ratio