Most have the serial number stamped into the passenger side frame rail on top up front by the motor mount. Some are lightly stamped and may be difficult to find. Scrub it lightly and shine a light across at an oblique angle to see it. There should be a star at each end and read 98EQ+5 digits
This is the only official vin for the truck so it isn't a problem that your tags are gone.
There may also be a tag on the firewall near the passenger side hood hinge that would have the number stamped on that, as well. But as Tim mentioned, the number stamped on the frame is the only 'official' number. Tags can be replaced or dummied.