This is my first RWD car (had two Hondas and one WRX) if I get the 3.9 gear it goes in the rear diff, and don't have to touch the tranny right?
Sorry for the dumb question, but I can get the gear installed for free if it's only the rear.

 

This is how it works in cars, in general

Engine --> Transmission --> Driveshaft --> Differential --> Axles -> Hubs/Rotors/Wheels

That should answer your question and provide you with more insight.