I have a 92 Z28 that I bought and I thought it had the stock rear end under it the 3.23. So I bought a new rear end to install on it and it is a 4.10 posi but when I got the car up in the air. I found that it wasn't a 3rd gen it is a 4th gen not sure what gear it is yet. I want to know why did they replace it with the 4th gen the 3rd gen had disc brakes and a posi.

 

If you bought the car used, there is no telling what anyone had previously done to it. Your question cannot really be answered.

 

One possilbe reason to install the 3" wider 4th Gen rear axle may have been to allow the high offset Corvette wheels to be installed. Were any "spacers" used? The size you show in your signature is confusing.... I have never heard of an 18.5 front wheel, or a 19.10 rear wheel. The C5 Z06 wheels were 17's in the fronts and 18's in the rear. Is it possible you have C6 Z06 wheels? There were 18x9.5 front, and 19x12.0 rear.

On the other hand C6 base model wheels (not Z06 or ZR1) were 18x8.5 front and 19x10.0 rear. The rears require large adapter/spacers on a 4th Gen rear axle assembly, they would require huge adapter/spacers on a 3rd Gen rear.

In any case, the way you show them in your signature is incorrect.

If the 4th Gen rear has drum brakes and no posi, it's out of a base V6 car, which makes no sense at all.