Dana-style axles have removable differential covers. This allows you to gain access to the inside of the axle without removing the shafts, carrier, etc. This also means that adding lockers or setting up gears must be done inside the axle. GM corporate 10, 12, and 14 bolt axles also use this design. The Isuzu corporate 8, 10, and 12-bolt axles, Toyota axles, Ford 9", etc use a 3rd-member style axle. There is no removable cover that allows easy viewing of the internal parts; but the entire 3rd-member (which houses the carrier and ring/pinion) can be removed as a unit. This allows lockers and gears to be set up on a bench, and then the complete assembly can be dropped back into place. Go look at the front of your rear axle (or the rear of your front axle) and you'll be able to see the large section that can be unbolted and removed.

BTW, yes, you have an Isuzu 12-bolt rear axle.