The 6 lug 2500s are just 1500s with stiffer torsion bars/rear springs. The 8 lug 2500s are the same as a single wheel 3500, just have a little softer torsion bars and springs. GM never made a "3/4" ton in these trucks. They are either built out of 1/2 ton parts or 1 ton parts. So for a 2500, there are 2 options for front differentials. The 6 lug ones are identical to the 1/2 ton ones. The 8 lug ones are identical to a 1 ton front diff. They are not interchangible. Transfer cases had a few options. If the 2500 was 6 lug and had a 700r4, 4l60, or 4l60e, it would have the same transfer case as a 1/2 ton. Most smallblock 8 lug trucks had this same case with a different input shaft in it to mate to a 4l80 or manual trans. Some of the big block (and diesel) 8 lug 2500s had the 1 ton transfer case, but it is not very common in a 2500. The rear axle is going to be a 14b semi float if its 6 lug, and is either a 14b semi float or full float if its 8 lug. (full floats are more common in big block and diesel applications, although they can be found behind a small block from time to time). The frames themselves are different between the 6 lug and 8 lug 2500s, the front differentials, steering knuckles, and upper control arms do not interchange. The lowers, however, are the same and can be had made of stamped steel or forged steel.