I grenaded my 10 bolt form my 92 Z28 and had to temporarily swap in a 10 bolt from a 86 or so Firebird while I put it back together. The 92 Z28 is a 28 spline and the 86 Firebird is a 26 spline. (I will refer to the spline count for ease of identifing which diff I am talking about, I'm not claiming that 26 splines are this way and all 28 splines are that way.)

While I had both diffs sitting next to each other, I notices some differences in the cases. The torque arm mounting pad on the 26 spline is not as tall as on the 28 spline, 4 1/2 inches on the 26 spline and 6 inches on the 28 spline. The 26 spline version requires large spacers that key into the bolt holes to raise the height to 6 inches. Without the spacers, the torque arm will not bolt up to this diff.

Also, if you drop the torque are bolts into the 28 splince diff, you can see them because the side of the case is open, there are only vertical webs between the top and bottom of the torque arm mounting pad. The 26 spline diff is not that way, you can't see the bolts because there is a large "X" cast between the top and bottom.

Also, there is a lot more casting webbing that supports the pinion on the outside of the 28 spline version. It looks a lot beefier.

Here is a crude drawing of what I'm talking about.

GM tried a few different case styles during the third gen series. The 82-85 were probably the worst. They redesigned the torque arm mount in 86 to prevent pinion bearing failures from case distortion by overtightening the torque arm bolts. They probably redesigned it again by 1990 when they switched to 28 spline axles.

There's no difference just because of the axle spline count. You can just as easily put 28 spline axles and carrier into an older case and it would work fine.

The late 80's and early 90's cases are just the better ones to use.

I know the spline count doen't make any difference in case interchangability or anything like that, I was just using the spline count to identify which rear end I was talking about at that time.