Ok, here's the skinny on the diffs (1/2 ton). The "10 bolt" and "12 bolt" diffs are the later salisbury (integrated housing) style rears (similar to a Spicer/Dana) used from '63-'87. The # of bolts refers to both the # of bolts on the ring gear and the # of bolts in the cover. The 12 bolt was used up to about '77, later on some 1/2 ton 4x4 models. The 10 bolt came in about '78, and was used in both front and rear applications. The 10 bolt is an 8 1/2" ring gear, and is considered to be about the same strength as a Dana 44. The 12 bolt is an 8 7/8" ring gear, and is considered to be in the Dana 60 range for strength.
Now your truck, if the axle is original, would have the 9 3/8" ring gear axle, with the drop out 3rd memeber. It was manufactured by Detroit gear and Axle (Eaton). There's not much being made for it as far as gears and hard parts, but bearings and seals are easy to get, and there is a 3.38 gear set being made. I think that maybe the std 3'90 gears are available too, but not 100% sure on that. You should be able to get any parts you need for it from the flaps.
