I have only rear wheel ABS. I stopped at the dealer and they want $100 to diagnose. I asked the servcie guy and he said it is most likley the rear diff sensor. I cost $30. He said not all the time does the speedo go out and sometimes the brake peddles pulses. I went with his suggestion.
We,ll see.
John

 

He's right, the speedo is not always affected.

 

i have rear wheel abs also I am about to the point of paying to find out just because I just want to know what it is.