Broncomanjoe

So my truck since I've owned it has had the abs light on and no rear brakes but every once in a while when it sits over night and its wet outside I'll take off and the right rear brake will lock up with very little pressure on the pedal. The guy I bought it from said something about the code being a rear abs module code? I don't know what this is but now that winters here I can't stop so before I kill myself or someone else any ideas?