1981 z28 10 bolt 8.5 any body know whether it is limited or posi, i know bouth were availble that year but i dont know what models came with which and when entering the vin at local dealer ship there computers only go back to 1990 any help would be great

There are a couple of things you can do easily. One is to jack up the rear wheels, turn one wheel and see if the other turns in the same direction (evidence of posi) or the opposite direction (peg-leg). You can close on the ratio by marking a spot on the driveshaft and the differential housing, then turn the wheel one turn. Count how many times the driveshaft turned and where on the clock it stops will give you the approximate ratio. There is no way to tell from the car that the rear is installed in. Posi rears were an option in all 2nd gen models. Z28's did not come from the factory with posi rears as part of the Z package.

Posi
Limited slip
Sure grip
Ect.
Are all basicly the same as they let one wheel spin faster than the other in a turn (slip) through clutches,cones,or such, but send torque to both wheels in a straight line for the best traction/performance. Posi is a "brand" name. Hope this helps.