do the gears from a 4th gen rearend fit into a 3rd gen rearend?

thanks

 

Yes. The 4th gen cars also have 10 bolt 7 1/2 / 7 5/8'' rear ends. The posi units and gears are interchangable.

 

One and the same 10-bolt. GM made no changes over the years, except ( axle spline count on late 3rd gen cars) and ( gear size ID from 7 1/2 to 7 5/8). Otherwise do what you want with parts as long as the carriers are the same..