You can try to reset the cluster by perform an instrument calibration. If I remember correctly, push the trip odometer and reset buttons in until the gauges react. The cal takes about 10 seconds. If it's still bad after that but, the speedo moves during the cal, it's a good bet the speedo is good and the problem lies elsewhere.
If that doesn't help try removing the IOD fuse for a couple minutes. It's in the fuse/relay panel next to the battery. This resets the body
control module (BCM) which controls most of the electronics in the van.
Still dead? It's either the cluster, BCM, or vehicle
speed sensor (VSS) on the tranny, or wiring. Since you're not complaining of shifting problems I would think it's likely not the VSS. If you go for the cluster, check the connectors contacts and solder joints closely. They are notorious for being subpar. An indication of this would be the speedo coming to life after you smack the top of the dash (common technique).
OBTW The dealer can determine the problem in minutes using a scanner.