The regular sensors are the same iirc. It's only a transducer mounted on top of a drive.

100% it's a dummy. 

Does the car have ABS?

Just found something that suggests that if it has, and was built after Nov 2002, the thing in the diff is a dummy sensor, and the speedo gets its info from the ABS sensors.

Sorry if this info is too late, hopefully not.

That is 100% correct, with abs dummy sensor, without abs working sensor.

I want to share the solution for the odometer problem.... I had the same problem. I changed 3 times the sensor. I replaced the tacho with another with no success. ...

I replaced ECU (with 93c76 modified) with no success. I checked the wires with ohmometer. Blue/white and brown/white was ok from the sensor to cabin. The black cable which powering 12v the sensor wasn't. Dont search for it in the cabin. The black cable is comming from the "waterproof" plug which is placed between battery and steering weel . I unpluged the plug and faced both black and green cables oxidated full with rust and water... I made bridged waterproof cables for these 2 (outside the plug). and replug the others... Now the odometer is ok..

I hope I help someone...