1997Sierra

I just noticed a problem with my abs brakes on my truck. It is a 97 Chevrolet 350 mpi, 4 wheel drive reg. cab longbox. When the rear tires spin out on gravel or snow it feels like my abs is clicking on. It seems to do this at low speeds and when traction is lost the problem can be felt. It stays on until I let off the gas or tap the brake. I do not know what it could be. Any help would be great.