To find it, you'll need a repair manual specific to your 1998 Dodge Dakota. The manual will show diagrams of the AC system and pinpoint the location of the orifice tube or expansion valve. Since the location varies slightly based on engine and options, a generic answer is insufficient.
Attempting to locate this part without the proper knowledge and tools is strongly discouraged. AC systems contain high-pressure refrigerant that can cause serious injury if mishandled. If you need to replace the orifice tube or have AC issues, it's best to consult a qualified automotive AC technician.