I read somewhere on here (unable to find it again) that the posi unit from a 4th gen would fit in a 3rd gen rear. I thought they were differant but I have never tried.

I have an 89 'bird with a 273 open, can get a 342 ( I guess ) posi from a 98 formula. Car wrecked and burned, not much left.

I dont really want to change the whole rear. Please let me know if this will work.

 

Depends on your Axle shaft spline count. 1989 was the change over year from 26 to 28 splines and that 2:73 rear is in question. If you have the same spline count, no problem using the 4th gen carrier & gear/pinion,

 

If they are 26 spline I can buy axles cheaper than a posi unit. I can get the rear from the 98 free