What differences are there with the diffs besides the gears on the inside and over all the 4th gen has a 4'' wider base. Can the 4th gen gears fit inside the housing of the 3rd gen with a shorter axle shaft? What gears are inside both diffs?

 

The gears are the same. The 7.5 was upgraded to 7.6 around '86 and the axles were upgraded to 28 spline in '90, but parts interchange between them as long as you use the right combination. Other than the over all width the 3rd and 4th gen rear ends are basically the same.