Thereare stamped into the metal on the vans and truck a serial number on either the back cross member in front of the taillights. Another area is under the doghouse. stamped on the lip where the doghouse for the engine bolts down. That you need to unbolt the doghouse and clean the seal away to find the number. It was on the rightside of my truck.
The crossmember I usually find them on the left one, but they can be on the right one.
The crossmember is is the metal brace from the side panel to the endgate structure.
I hope this makes sense.