[1981zapper]

My car original came with a 3.42 open rear end, the previous owner said he changed out the 3.42 differential and installed a '78 3.73 posi differential. The other day I had the car on jack stand and when I spin the driver rear tire the passenger rear tire spins in the opposite direction, I never had the cover off to look inside but I assume its all stock gm stuff, I was told that my clutches are worn out? I don't understand because, if I am at a dead stop I can squeal tires and if I manual shift it from 1st gear to 2nd I can chirp the tires

[lmsport]

COuld be a worn out clutch posi or a like new TruTrac.

[S10 Racer]

Things to try:
1. Try turning the driveshaft instead of the tire.
2. Put one tire back on the ground and try to turn the other.

[big gear head]

'81 Z28? If it is then you have the 8.5 corporate rear end. That will have the Traction Lock differential. It's very comon for the clutches to wear and loose their preload, which will make it act like a standard differential when you try turning the wheels like that. The only way that you are going to know what you have is to remove the cover and look inside. There is no other way to be sure. There are after market reproduction clutches available for these now at about $200 a set with shims. They are no longer available from GM. You can also shim the clutches that are in it if they are not worn too much.