Check out the Drivetrain Forums. keep in mind "positrac" is a GM term, Trac-loc is the Ford term.. Trac-loc will yeild you more results when doing a search on it.
This is true. "Posi-trac" and "Trac-Loc" are both brand names for what is commonly called a "limited slip" rear end. Its misleading because neither is actually a "locked" rear end.
However to answer your question.
Yes, its been done many times. I myself have done it. Bear in mind that while you are searching the yards, that not all mn12's got the track-loc, V8 or not. The most accurate way to determine if the center section ("pumpkin") is a limited slip is to check out the metal tag that is held on by one of the bolts (usually near the top) that holds the rear cover on. If the code has an "L" in it, its a limited slip. for example "3L27" is a 3.27 Trac-Loc.
The easiest way is to is to check the rear end code from the tag glued in the drivers side door jamb. The codes are on this chart:
Other good tech articles to read:
Keep in mind that it is possible that someone could have changed rear ends before the car made it to the salvage yard so the door jamb, though easier, isn't as accurate as the tag on the rear end itself. When/if you find one, be sure it is low mileage, and even then you probably ought to have it rebuilt. The first one I installed lasted about a year before the clutches wore out and it acted like an open rear end. The second one I installed is still good. (it was much lower mileage when I installed it).
Results? Great, I love it. Car launches much better with it.