98 up will have an input speed sensor on the housing on the driver side, right by the shift linkage. If you pull the tailhousing off, you will see a 4-lobed rotor.
97 down will have no input speed sensor, and if you pull the tailhousing off, you will see a 2-lobed rotor (only one triggers the sensor, the other is there for mass balancing)
The part number on the sticker on top of the whole unit will also be different, but I don't know those numbers off the top of my head... yet.