Hi Dodgerocks, love your nym!
To start at the simple end of the spectrum, out of curiosity, are the correct size rear tires on your van? They should be listed on the data plate decal in the driver's door frame, if the conversion company didn't remove it.
What is the mileage on the odometer? If something is worn, that might help point where to look. A 1997 might not even have a direct mechanical connection to the cluster; that year is late enough it might be electronic. If you see this post, what do you think Reed?
Does your van have cruise control?
Can you see what sort of connections there are at the back of the instrument cluster if you peek up under the dash? What does the sender mounted at the rear of the transmission, and its cabling look like? Try tracing the cables back towards the front of the van, especially if there's a big one, about 3/8" in diameter. A couple of pictures would help us greatly, if you can get at those areas.