Need help with ABS problem

My fiancee has a '01 Blazer, recently thew ABS system is faulting somehow. While driving and coming to a stop, once I pess the brake pedal, there is a very loud grinding or banging noise, as the vehicle stops the peddle pushes back toward me. sometimes at this point the brakes dont engage or engage very sluggishly. I gave replaced all the pads and still the problem exists. if I disengage the ABS system by unpluging the wire harness on the unit, the car brakes fine. Any Ideas on fix and on cost? Thanks in advance.