I have an 2003 Dakota and my ABS seams to work horribly.

In a controlled environment "Snow covered parking lot" I apply the brakes and the abs kicks in on the front tires but the rear tires just seam to free roll and dont lock up or do anything but roll. It seams as my braking distance is much too long. The brakes have been replaced last fall with 15,000 kms on them. I'm going to take it to a shop because it just seams far to unsafe, but I was wondering if anyone else has this problem.