can i bolt a 2-series gear to a 3-series posi? do i need a spacer or something? how do i tell if i have 26 or 28 spline on the 89?

Only works the other way around.

Won't happen; to do what you want to do would require a spacer that took thickness out, instead of adding it. Pretty tough to do given the present state of metallurgical technology.

Only way to know what splines you have is to pull an axle and look. Odds favor it being 26, but not heavily enough to buy parts based on that.