I have a 69 Z28 that i have been rebuilding, for some reason when i bought it came with a 10 bolt rear end. I located a 12 bolt posi unit with the correct date code that came out of a 69 camaro also. I mounted it onto my car the posi unit seams to work perfectly except on long sweeping curves on the highway it has a funny feeling like the axle is moving side to side, can anyone help me with this problem?

Double check the mounting bolts after driving a short while. Also, does your car have a rear swaybar? Most Z28's did, but if not that might be a needed item.

The clutches in the center section are more than likely hanging up. Change the oil and don't forget the posi additive. Although, I wonder if the (one post) OP is coming back?