You are giving me motivation...bad, expensive motivation... LOL
To answer someone else the speedo only needs the feed back from one sensor to work. The ABS needs 2 sensors to work. From what I've been reading a lot of people put sensors on the rears and that's where they stop. You need all 4 for traction control to function.
What will putting all 4 sensors on one axle do? Just shut the light off but not really allow the traction control system to work?