just because he has a one wheel peel does not mean he did not have limited slip.. the limited slip will wear out over time. If you do doughnuts it will wearout even quicker.. on the door trim tag you will see under AX either a number 8 or a letter.. if you have 8 there then you had a open rear end. I highly doubt you had a open rear on a gt.
As long as you had limited slip from the factory then all you need is the clutch pack rebuild kit. there is two kits out there. the 50 dollar one you have to shim yourself, then there is a 100 dollar one that comes pre shimed.