1998 Chevrolet Astro Speedometer and antilock dash light

It has done this a few times in the past but never for this long, or this bad. When I was driving home, the speedometer drops and jumps around no matter what speed I an traveling, which in turn causes the van to sputter and cut out, if you will. That is something that it has done on occasion, but last night it would drop from 60 mph, down to 10 or 15 mph and 10 minutes into our 25 minute drive home the Anti-Lock dash light came on when the brakes had not been in use due to a straight stretch of road with no other traffic. I need to know what is causing this so that I can remedy the problem, for it is a family vehicle.