Looks like the first post glitched...it's not showing up for me.

The axle spline change was mid-year of '89, so anything '90 and up will have 28-spline axles. Some '89s will also have 28-spline. '88 and older were all 26-spline. This all for 10-bolt rears....I believe the 9-bolt had the same spline count the whole time it was used in the 3rdgens.

In general, yes. There's a possibility that a few 26ers might still have been around in 90; but not many. You can almost bet that a 90 will be 28.

so a 1991 is DEFINETLY one, right?

yep

Correct but chances of finding a 89 with 28 spline is rare. It depends on the production date of the car.

9 bolt axles are not interchangable with any 10 bolt but I believe they are 28 spline.

So my '91 has 28 splines?

LB9GTA

If it doesn't, someone installed an earlier version diff under your car.