That is not an OBDII engine, but rather a Ford EECIV system. It has a connector that you either use a device that flashes, beeps or a fancy one counts and generates codes. Poor folks like me use an analog multimeter to count the pulses of the system codes. No idea where it is located. On my '91 Ranger pickup and Dad's old '84 LTD (long gone now) the connector is in the RH side of the engine compartment near the battery, no idea where it is on a chassis like yours.
You need a manual to tell you what the codes mean.
'03 Ram 2500 CTD, 5.9HO six speed std cab long bed Leer top and 2008 Bigfoot 25B21RB.. Previous, Being repaired and will be sold, is a 2008 Thor/Dutchman Freedom Spirit 180. SOLD - 2007 Winnebago View 23H Motorhome.