2005 Pontiac Montana caliper locking up

Our right passenger caliper keeps locking up We have replaced it 4 times to rule out a bad part. We have replaced the pins, pads, soft brake lines and the rotor. It will drive fine and as soon as you hit the brakes hard they will lock up and smoke. Sometimes the ABS light will come on when they lock up and sometimes not. Our mechanic said we need a new ABS unit which includes a module and pump. It costs 1,000 and can only be ordered from the dealer. They said when they release. The bleeder screw on the ABS unit the calipers delease. If this will solve the issue I will find a way to get it done. Could there be something else causing the problem? Thank you