First off we have to remember that these are also called idiot lights. They may just be disfunctional. If it feelsas if something is loose it could be in yuour rotor , caliper, pads.What was worked on last in that area is usually a good place to start. As well as the abs is a brain that makes the brakes let go before the car stats to lock up and slide so if you coyld actually slam them on at maybe 20 or so in a safe area and see how the brakes feel like if they actually do lock up and maybe one wheel in the back will stop turning and leave a mark on the pavement it is not doing its job.I believe in a case like this a diagnostic test would be ur best bet. For this particular thing it usually runs between 25 and 40 dollars. Burt dont let them talk in to all kinds of stuff SOMETIMES THESE PROBLEMS CAN BE RATHER COSTLY BECAUSE OF THE COMPUTER BUT ONLY BECAUSE THEY WILL OVER CHARGE SO CHECK IT OUT A COUPLE OF PLACES