That suggests there's mechanical damage to the motor, the gear, or the track that moves the seat. That would likely require that somebody needs to unbolt the seat to do a visual inspection. Before you make any arrangements with your local mechanic, do some housekeeping & pull out all the clutter and garbage under the seat. Maybe you'll be lucky enough to find something stuck where it doesn't belong causing your problem. It's unlikely that there isn't still irreversible damage to the aforementioned parts, but it's still worth looking into and your mechanic will appreciate not seeing any of your dirty little secrets tucked under the seat.
Either way, you're probably in for the cost of parts & labor. Getting a salvage yard seat to swap out mechanical parts is going to be a likely avenue of repair. (If you have an 01-07 van, it's in the realm of old where parts like that are becoming more obscure all the time and were never that plentiful in the first place.) If you have a common interior color like grey, your mechanic might be able to just swap the seats out entirely.