My '84 book ('83 should have used the same codes) shows a Dana open axle, 3.07 gears, and 6250 lbs capacity. IITC that should be a semi-floating Dana 60...do you have hubs/axleshaft end caps sticking out from the brake drum a few inches?
The semi-floating Dana 60 is a rare bird especially behind a diesel (it's more of a light-duty F-250 thing), but I've seen a couple on '83 and '84 trucks, so I can say with authority that they do exist...even though most semi-floating axles are 5-lugs and found on half-tons.
Full-floating axle:
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Semi-floating axle:
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