Faulty sensor? I had a similar problem on my lorry at work a few years back. ABS light would be on - brakes no problem. First start up after break - no ABS light on. First stop after this and abs would kick in on a completely dry surface and you'd struggle to stop.

On the wifes rover there was no light coming on, but the abs had a habit of kicking in for no reason when coming to a stop sometimes. The reluctor ring on the cv joint was cracked and when I came to change it there was a big lump of rust stuck to the sensor face. I think that might have affected it as the passenger side ring is cracked and braking is fine now.

I'm assuming your grinding and vibration is the abs kicking in.