Abs wheel sensor question?

Ok my abs light comes on alot, especially when it rains. That means I have a bad wheel Sensor right? Well how do I know which one to change? Or can I clean them all? Can a dirty sensor be the problem? the light usually comes on when I first start the truck and when I come to a dead stop, put it in park then back to drive the light goes off. But other times it will come on as I'm stoping and the pedal vibrates. I have a 1998 s-10 blazer.