Sounds like someone changed the rear end gearing...
In your case I agree, the gauge itself in the cluster, because of the "incedent" you mentioned. You know the cause, where, when, how and so forth. It has been off since that time.Except, we've owned the van since 99
I'm convinced it has to be something in the cluster, because the VSS is accurate and the cluster is supposed to simply display the calculated speed.