rear end identification

Im hopeing one of you can help me.
Im haveing the hardest time finding out about the rearends they used for my 69 cuda fast back.
I know its an 8 and a quarter with 3.23 gears but i have another one in a 68 that acts like a posi traction.
Lights um up all the way down the street.
Im thinking of swaping the rear end thats in my 69 for the one that came out of the 68.
My question is did chrysler make a positraction in the late 60`s for these cars /,and if they did how do you identify it?
Or is this what they called a limited slip differential???
Thanks any help would be over whelmingly appreciated!
Machin head Dan