Both terms mean the same thing basically. The 1990 Mustang GT has Ford's Trac-loc limited slip. Though it's entirely possible that 22 years later the clutch packs/discs are likely too worn to really function that well, Trac-loc's generally require a rebuild after around 120k miles or so to remain effective. The good news is that rebuild kits are pretty cheap.
There is a difference between a limited slip and a full locker.