Someone correct me, i have a 96GT with the stock rear end, isnt it posi, i get 2marks

[The Deuce]

Or the almighty 2L73, yeah baby, highway gears! (ugh)

Posi is the GM term for it. They had it first, now its kind of like Kleenex. I think they even Copyrighted that name. Track-lok and Posi are both names for limited slip.

If one tire is lighter in a corner, would be a weight thing. Once you get them both spinning, they will leave the same mark in a straight line. Around a corner the inside tire will leave a lighter mark because there is less weight. You can see this effect exaggerated with a Mini Cooper, which will pick the entire inside rear wheel off the ground.

If you think you Limited Slip is shot, a straight line test is the only real way to be sure, without a "real" test of the part.