Is It A Posi?
Everyone's heard the tip about checking your vehicle's rearend to check for a "posi-traction" or "limited slip" unit as opposed to an "open" unit....and that is to jack both wheels off the ground and try to spin one tire. If the opposite tire spins in the same direction, it's a posi. If it spins in the opposite direction, it's not. However, this is NOT an entirely accurate way of correctly determining the existance of the posi.
A posi that is set up right will cause both wheels to turn the same direction with the trans in N. When the trans is in P you should not be able to turn the wheels. The problem is that when the clutches wear, the unit loosens up and begins to act like a "open" or standard unit. In this case the wheels will turn in oposite directions with the trans in P. The only way to be sure is to remove the cover and look at it. If it has the preload springs, plates and clutches, it's a posi. If it is a posi and it is acting like a standard unit, you need to have the clutches replaced and the preload reset.
The only way to know for sure it to take the cover off the rear end and look at the carrier! Look for the two plates with four springs between them, or the "s" spring in the later 10-bolts. Even if both wheels were to spin the same direction in the above scenario, it could mean that the rearend has welded itself together from too many one-legged burnouts...or it might even have a racing-only spool installed. NOT the ticket for a street-driven car.
According to the 1970 service manual, for a new posi-traction differential, the torque required to rotate one wheel while the other wheel remains stationary should be 70 ft. lbs. minimum. For a used posi differential it should be no less than 40 ft. lbs.
hope this helps
taken from oldsmobility.com