I have never in all my years working on and playing with these trucks seen a 12 bolt in a 3/4 ton! Only rear offered in 3/4 ton was a 14 bolt full floater, and a 14 bolt semi floater.
12 bolts are not that much stronger than a 10 bolt. You will need a crossover u joint, The cross width is shorter on a 12 bolt, and the cap diameter is smaller on a 12 bolt.
All Chevy 1/2 ton 2 wheel drives are the same lug pattern.