I have a 1996 z28. Its an automatic and i have recently noticed the posi wasnt working correctly. i raised the rear end up on jacks and started the car and put it in drive. the passenger side wheel was spinning nice and fast while the drivers side was barely spinning, it looked like something was trying to hold it back because it was very jerky. any idea what might be wrong? maybe the clutch packs are bad?

 

It takes some torque applied to the axles to cause the posi clutches to engage. If you are freewheeling on jackstands, it may not be transmitting enough torque to lock it up.

Take it out on the street. Stop. Pedal to the floor. Stop. Walk back and count how many black stripes there are. That's the best way to test it.

 

i forgot to add. i tried burnin the tires a little to see if ibhad to burn marks. only one. wheb i jacked it up and put it in drive. everytime the drivers side wheel would cut out my rpms lowered almost like it was mis firing. but when the brake was pressed it rpms went to normal. i tired giving it more gas but engine kept cutting out so stopped on that part

 

Could be a sticking brake caliper on the one side.

Also, drain and replace all the fluid in your rearend; remember to add a bottle of the GM limited slip additive. A lot of times this will help revive a posi that's not working well.