Hey, Will. Did you call Astro? He knows exactly what years. I think ALL 4.3L powered 2WD trucks(after'88) have the 28 splines. I saw on a website once where it told what year they switched. I believe all 93 2WDs with v6s have 'em also.
1989 S-10 4x4 4.3L/700R-4, 3.08s, 31" Sport Kings
1989 Blazer 4x4 4.3L/700R-4 3.42s,235 Sport Tracks
1985 S-10 4x4 Soon to have 3504BM/TH350/D44/14FF, 35" Boggers