This is really easy to fix, but you gotta get under the car. The VSS (Vehicle speed Sensor) is a blue sensor on the driver's side of the transmission, right at the back near the drive shaft. It's held on by one bolt; disconnect the electrical connector then remove the bolt. Twist the sensor to remove it, there's an o-ring holding it in. The old gear is held on with an e-clip (same kind that comes with the new gear). Pop off the e-clip with a screwdriver, and install the new gear & e-clip. Reinstall the sensor and you're done!
Be aware that your speedometer will be closer, but not correct. There isn't a speedometer gear for 3.55's in an AODE that is 100% accurate.