Antilock brakes not funtioning properly on "hard" stops

I have a 1997 GMC Safari 2wheel drive (181,000 miles) with the 4WAL antilock brakes. I keep the vehicle well serviced. When coming to a "normal" stop, the brakes work perfectly, coming to a smooth, safe stop. However, today I had to execute a "hard" stop at a traffic light, (traveling about 35 mph) which engaged the ABS. (It had rained earlier in the day - road was wet, but no standing water). Prior to this stop, the van would have stopped well before reaching the intersection without problem. This time It jerked, made lots of "bumping noises" and felt like trying to stop a locomotive! The van finally stopped in the middle of the intersection. None of the ABS warning lights have flashed. No prior ABS problems during hard stops. I then tested the van on a secluded road. If I quickly "pump" the brakes for a quick stop short of engaging the ABS, the van stops as it should. Hit the ABS, and it keeps going and going while the ABS bumps and thumps and does not stop as it should. There is a lot of "feedback" to the brake pedal. One instance, after stopping with ABS, the brakes would not release unless I pressed the brake pedal rather firmly all the way down, then it released. Although I have not measured them, the brake pads, rotors and drums appear to be within spec, although there is some normal wear. Brake fluid level was in the normal range. I would appreciate any guidance you could provide towards correcting this hazard!