As mentioned above, when it came to the front and rear axles, all evidence indicates GM did not stop use of them at XX year...if they had a 12 bolt with 3.73s at the end of 1980, but a 10 bolt 3.73 was to be THE half ton axle starting in 1981, they would use up the leftover 12 bolts into 1981 or beyond if the parts were there. Functionally no different. Not sure how that played with emissions, but it was certainly a factor with engines and transmissions (the fewer options/combinations available, the fewer EPA tests the manufacturer has to conduct, plus economy of scale in general) one would hope the EPA isn't so crazy that each half ton axle would have to be tested simply because it was a different design. The transfercases are a different matter...such as the 205 being the only case offered in 1980, and the last year it was used in a K5. 208 only from '81-88, then 241 to 1991. You hear all sorts of anecdotes about how someone knew the original owner of a K5 had a SBC 400 that was factory in a 1983 K5, but you also used to hear people argue about 12 bolt fronts, or that they could get everywhere in their 2WD truck that a 4WD one could go. 
(most of those people seem to be gone it seems) Doesn't mean it was fact just because more than one person made XYZ claim.
Not meaning to rankle anyone, but I doubt any 12 bolts or Dana 44's made it to the mid 80's. All it would take is perusing the shop manual for those years to see whether the service instructions are there for that axle or not. Go with an '80, and I wouldn't be surprised at all to see the 10 and 12 bolt service instructions. An '85, I highly doubt it. But with the crossover years, it would be easy to make one fit as if it were factory, I'm sure.
(most of those people seem to be gone it seems) Doesn't mean it was fact just because more than one person made XYZ claim.Not meaning to rankle anyone, but I doubt any 12 bolts or Dana 44's made it to the mid 80's. All it would take is perusing the shop manual for those years to see whether the service instructions are there for that axle or not. Go with an '80, and I wouldn't be surprised at all to see the 10 and 12 bolt service instructions. An '85, I highly doubt it. But with the crossover years, it would be easy to make one fit as if it were factory, I'm sure.
