Rear Locker or Limited Slip?

[Dcraine101]

What's up guys I recently bought a 83 c10 and I'm trying to figure out if it has posi. But I jacked up the rear end and it's weird. If I spin the passenger tire slowly the driver side spins in the same direction but if I spin it fast the opposite tire doesn't spin! And sometimes it'll even spin the opposite direction! So I don't know if it's an open diff or not! Any help would be appreciated! But no matter what if I spin it slowly forward they both spin in the same direction

[Captkaos]

Pull the cover and inspect, probably needs new gear oil anyway.
Have someone hold the other side.

[87max]

what capt said, its probably open but just jack up one side if its a locker it will engage and you won't be able to spin it. if its a slip you will hear it try to engage. factory slips (some aftermarket) require wheel speed to engage the clutches you will not be able to do that by hand.