It's usually a torque convertor issue or a worn overdrive band inside the transmission. That
happens when the transmission wears out. In your 27 year old truck, I'm sure you have over
150K miles. So I would say it's time for a rebuild. Visit your local mechanic or trans shop in
your area for a proper diagnosis. But a rebuild will cost you more than the Van is worth. I
usually recommend a used transmission, but that one will be equally as old. So I would
continue to drive the van till you can afford to purchase something a little newer.