He is talking about emissions testing on the dyno where they need to see the mph for testing purposes. You need to get under the truck right near the t-case output and check to see if there is mechanical or electrical sending unit. Electrical would of course be wires and mechanical will be a thick cable like a bike shifter with a spinning cable inside it. The mechanical types use a removeable plastic gear and by counting the teeth on the gear you have you can do a little math and check to see if the gear tooth count you need is available from isuzu used on a different application (oem bigger tires). There is post around with some part numbers.

If you have an electrical speed sensor then you should be able to use the electric unit. the way they work is by chnaging the frequency of the signals sent to the ECM and add or subtract a ratio to make up for the tires. multiply by 1.123 for instance. In that case it shouln't matter what brand of vehicle they all work in similar fashion. Maybe you will have to pick the right wire off the sensor, dunno, the manufacturer might be able to help and more poking around the net.