I am wondering the same thing. I have a 76 CJ5 with AMC 20 with a big N stamped on the top of the housing. I tried the same thing, jacking it up and found exactly like you did. I opened my dif. cover and thought it looked like an open dif.,but after seeing a few in catalogs I am not sure. ????is the carrier more squared off on a trak-loc and more rounded on a open????
76 CJ5 304 "its a JEEP thing, I hope you understand"