you have a limited-slip, aka Trac-loc per Ford motor co...28 spline piece of garbage. If you have the means replace it with an Eaton or spool and rock on.
actually it is not a piece of garbage. It holds together as well and better than most rearends. There are many that have put a lot of hard launches on them before / if ever they come apart. I did upgrade to 31 spline rather than wait until it breaks. Talk to the GM guys about their 10 bolts.
Think of a limited slip differential like a type of "hardware" traction control. The electronic traction control would be software.
The traction control is what messes with the fuel/spark and brakes to help keep you from doing 180's in the middle of the road.
A line-lock is for doing a burnout for drag racing. You push down the brake pedal, activate the line lock, then release the brake. What it will do it keep the front brakes clamped while the rears can let go, so you can do some sweet burnouts while remaining perfectly still. Normally, you would have to keep your foot on the brake enough to keep the car from moving, which would also make the rear brakes start to clamp down. As you can imagine, fighting against the rear brakes like that aren't the best for their longevity.