A Z71 posi is a gov-bomb they SUCK in a huge way. The car posi is way better. The difference between them would be years not car/truck. In '89 I believe they went from 28 spline to 30 spline.
The gov-bomb is marginal on snow or mud but violent and scary on pavement till it breaks and it will.
If an open diff then it should be fine.
There is some chance the carrier bearings could be different. I know when we put aftermarket posis in we end up needing new bearings and cups and I thought those were "truck" parts, carriers are interchangable though.