Check your RPO Codes to see which rear you have (usually beings with GU, like I have the GU2 which = 2.73). This will give you a good idea as to the exact ratio, which maybe linked to your specific posi unit. I know my 99 z/28 has the torsen unit, which was supposedly new for 99'. I think the 98 and lowers and the auburn but not sure. Do some research on here and see what you can come up with. Good luck.
jack up the back of the car and with it in park or in gear see if you can spin the drivers side wheel forward, if the passenger side spins the opposit direction, it is a torsen, if neither spin, take the car out of gear and you will see when you turn the drivers side forward the passenger side will spin teh same way and you know you have a auburn
run mineral oil with additive in both applications