So your front sensors are F'd and it thinks the wheels are locked and releases the brakes. Typical GM safety issue and you'd better be ready at times to throw it in reverse before you get the guy at the lights in front of you. Pull the fuse.. I believe it's a 50 amp. If your hubs bearings aren't gone, which is a usual cause, but the '98/20 sensors always went bad on their own with hubs still good. I presume it's been on a hoist and the front wheel/hub assembly have had the wobble test. I've been driving my GMC plow truck that way for over a decade, put the fuse back in = no brakes.