ABS releases the pressure in the brake line by using a solenoid servo and some check valves. The fluid goes into the servo, then is pushed back into the brake lines when pressure is restored. Obviously, this make the pedal vibrate.
If you are on wet ice, you get some noise from the wheels and brake pads, too.
ABS has been around for a long time now, always fails "simple" as if it wasn't there. It is designed to leave normal brakes when it fails, not to lock things up, for obvious reason.
Poineered by Chrysler in 1978, I think.
Rock hard pedal sounds more like a failed booster or broken line to me -- makes the brakes very hard. Or stuck calipers -- the ABS will attempt to match wheel speeds, so if one wheel has brakes and the other doesn't, you can lock up a tire and the ABS will work. Very hard pedal, as one or two pistons aren't moving the in caliper.
Get the brakes checked, front and rear. I suspect a mechanical problem, not an ABS problem.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!