"I am in need of an exhaust manifold for my 1990 YJ, saw they have a 89 and 91 should be the same huh?"
I don't know Paul. If I want to check something from home, I'll find the part and application on Advance Auto's website. When I want to see if another year fits, I add that year vehicle to my garage.
If I'm goint to the yards anyway, I'll ask for an interchange list while I'm at the counter. The bigger yards will print off a list of the vehicles on hand, that might have the part.