I've never seen a Dana 20 transfer case in any GM after 1972..only a few straight six 1968-72 K10's...didn't know they still used them up till 1974..
I had a '75 2wd Blazer that had a 250 six and a Tremec 3 speed factory..first and only one I ever saw!..usually they used Saginaw or Muncie 3 speeds..