One legged

[Roger63]

Is there a way to turn a 8.75 open to a posi without replacing the entire center section?

 

[badmopar67]

change the guts to a sure grip still not cheap. but cheaper if you can find a used one in good shape.

 

[Dako]

If you mean without 'removing and re-installing' the center section, the answer is no. But you can swap out the open carrier with a limited slip unit, retaining the current gears and not touching the pinion gear at all.