If you have a factory v8 car of the vintage in your sig, then you have a 8.8 rear. Probably a 2.73 or 3.08 gears. I believe that 2.73's are most common.
If it is the stock rear that came with the car then yes it has traction lok. But depending on how much abuse the car has seen over the years it might need to be rebuilt. The rebuild kits are about $45 and available at many places over the internet and I would think at your local Ford dealer's parts dept. If you are going to change gears then now is the time to find out if that t lok needs a rebuild.
Both tires should spin when you launch straight ahead. When you turn corners the clutches in the t lok disengage and allow the wheels to turn at different rates, the outer wheel turns faster since it has more distance to cover while turning the inner wheel turns slower than the outer. This is with respect to the corner of course.
That'll give you an idea of where it is need of a rebuild. Personally I would ask how much to rebuild it while they are putting the gears in there. If it isn't too much more I would have it done. Here is a site that has a bunch of great info on installing gears and rebuilding t loks.
It will be some more labor cause in a gear install you just take the diff out you don't have to disassemble it. With the t lok rebuild you have to take the spider gears out and then the clutches and steels and then put it back together. Alot of guys have done them themselves so it isn't that hard. I would think about and hour maybe two of labor.