To be absolutely certain, you should:
* Consult your owner's manual: This is the best resource for the specific location in your vehicle.
* Look for a label: The ECM may have a label identifying it.
* Check online forums: Search for "1998 Dodge Ram 3500 ECM location" on forums dedicated to Dodge Ram vans. Pictures from other owners can be very helpful.
Remember to disconnect the negative battery terminal before working near the ECM to prevent accidental damage.