I did some additional research on this and it looks like the 28 spline 7.625 carriers did come out for the 1990 model year. There are several discussions at thirdgen.org on this topic.
When talking about g-bodies then, you should never find a 28 spline 7.5/7/625 from the factory. When comparing 7.5 to 7.625 rears in the g-body era, everything should be compatible. If you put a 7.625 ring gear in a 7.5 rear, you have a 7.625 rear. If you put a 7.5 ring gear in a 7.625 rear, you have a 7.5 rear. Is there really a difference then between a 26 spline 7.5 carrier and a 26 spline 7.625 carrier in any given year? Do they have different part numbers? I don't know, but would be interested if someone knew the answer for sure.
Is it possible you might find a 28 spline carrier in a g-body? Sure, but it is highly doubtful it cam that way from the factory since the 28 spline carriers didn't come out until late 1989. I'm sure some have swapped 28 spline carriers from F-bodies to take advantage of the 28 splines and supposedly a better posi unit than was offered in g-bodies.
Also, I was not able to locate any aftermarket gears being made in the 7.625 diameter. All seem to be 7.5.