Two things to check. Now, I'm not particularly intimate with the Dodge van, but a lot of mopar products of this era have a speedometer that "plugs in" to the printed circuit board. The "plugs" look eerily similar to the pins on the back of our 70's-era gauge clusters, but are much longer.
I've seen these pins get corroded and since they are longer, they can also break off losing contact with the PCB. IIRC, it takes three or four pins to run the speedohead, if one breaks, the odo will still work, but the speedo may not. (or vice versa)
Check that first, but failing that, you may need a new speedo.
Bloody Mary, Full of Vodka, Blessed art thou among cocktails....