How to tell if it is a posi trac????

I have a 91 F-150 2wd with bix 6 cyl and a 5 spd. I was wondering what is the best way to tell if it is a positrac rear end or was it an option that year. If you raise both rear wheels and it is a posi what will the other wheel do when you spin one? Thanks.

I had a friend with a ford that had a positive traction rear end and it made a wining noise when it took off.If I am not mistaken I believe that both wheels would turn the same way if you lifted them off the ground and turned one side by hand.

There should be an axle code on the door jam area. I know both the F250 and Excursion have a code.
The truck is single the suv is dual.