Abs sensors?

[Stic-o]

I have a ABS light on in my wife's truck. It seems to stop just fine and the ABS is working. How can I tell which sensor is bad? The rear needs brakes but I don't that would cause the ABS light to come on. So I'm pretty sure it's a sensor, I just don't wanna replace all 3.

 

[RiverRat]

Only thing i could recommend is a scan of the abs module.