It's kinda funny that I'm getting and answer now since just today, actually, I may have figured it out. But I'm always on the lookout for better ways!
First, yes your definition of transmissibility is correct and is exactly what I'm looking for. Second, I was looking to get this from either a response spectrum analysis or a harmonic analysis. I wasn't aware that this was something you could get from a modal analysis, so I'm very interested in that method, if you have a way!
The method that I have solves a harmonic analysis using a displacement load at the boundary conditions and measuring the output displacement at a node divided by whatever input displacement that I set (typically I'll just use 1 inch) using the quot command and then output the results onto a graph. If you see anything wrong with this method, please let me know. And thanks for responding! I appreciate your help with this!