Not sure if anybody else chimed in on this, I didn't feel like reading through all 14 pages.
"1500/2500s used a semi-floating rear axle and 3500s used a full-floating axle."
Any 1500 HD (F44) or 2500 LD truck with a 14-bolt and 6-lug axles was semi float. Some 8-lug, 8600lb GVW 2500 trucks had semi-floating axles, many (I would guess most) had full-floating axles. I've seen two completely identical K2500s from the same year with the same options, engine, everything, but one will have a SF one a FF. Mine happens to be a SF (see sig, it's an 8-lug, 8600lb K2500).
I don't know whether this was something you could order from the factory, or just whatever GM decided to throw in there. Seems strange to me.