If the rear ABS sensor is bad, it will light the ABS light BEFORE the vehicle moves. If the light comes on at 12 MPH on the first drive-off after start-up, it means the system failed its dynamic self-test. If it comes on intermittently and without pattern, it might be any one of a number of things.
Get the code read. A cheap OBDII scanner will not do the job. See the referenced thread in the previous post, it has more information in it.
Last edited by projectSHO89; 08-11-2012 at 08:49 AM.