Since I worked for the GM axle plants iin product engineering in the mid to late 80's and in 94 for AAM when they were seperated from GM, I should know the answer to this question. I believe the 12 bolt axles have a larger ring gear (8-7/8) vs 8-1/2 for the 10 bolt design. There could also be a different hypoid offset. Also, the axle shaft splines into the side gears are likely different. Don't remember about the wheel bearings into the housing or the fit onto the axle shafts. So, in a nut shell, not interchangeable. In later years, GM got smarter and upgraded the 8-1/2, 10 bolt and eliminated the proliferation of the 12 bolt. Even more recently, the 8-1/2 for passenger vehicles, specifically the F-car went to an upgraded 7-5/8 rear axle with better steel and heat treat technology. There's no question a 12 bolt is a better (stronger) way to go!