I'm not sure I understand what you mean by "3rd member design". However, RWD and 4x4, not AWD 1500 GMT-900 SUVs with the 6.2 and 6.0 Hybrid received the 14-bolt 9.5" semi-floater rear axle versus the 8.6" used by the 5.3s and the 6.2s with AWD. Within this same generation it is a plug and play affair and one does not need to open them up (parts replacement) unless they want to change the gears.
I misread your previous statement. You were talking about used price of about $400 and drop it in in the afternoon and I did not pick up on you were talking about a complete axle. When you said drop in my mind went to a third member differential, like a Ford 9" diff. Instead of building the gears in the housing a Ford third member is a complete unit that just unbolts from front of rear diff and lifts the whole gear assembly out and then "drop" in a whole different gear assembly/section. Since I have been away from shops for a while I thought something weird happened and GM was making the 9.5" semi float set up as a third member. I am old. Need to read better before replying. My bad.