Have a 2004 Chevy Impala, want to put in transmission from '02 Impala
 

Hey guys.
I have been researching my vehicle & I know my engine (3.8L) & my axle ratio (3.29) & I am looking into buying a used or rebuilt transmission. 'Cause y'know, buying a new transmission is so expensive I may as well buy a car. Anyway, can anyone explain to me why I can't use a transmission from an older Impala (I am looking @ an '02) if the engine & axle ratio are the same? I just want to understand if I'm barking up the wrong tree.
Thanks sincerely,
-M