Check to see if the cable is spinning when the van is moving. You only have to drive it a few feet to see it. If it is not, suspect the plastic gear in the transmission. Use one of the same color so it has the same number of teeth. The number of teeth might be molded onto it too.
If the cable is spinning, suspect the speedometer.
If you have cruise control with the large servo and two speedometer cables, one to the transmission and one to the speedometer, check to see if the lower one is chewed up on one of the ends. If it is, the servo is bound up and must be replaced too.