Here's why it's so complex and what would be involved:
* No pre-existing fuel injection system: The engine wasn't designed for it. You'd need an entirely new fuel rail, injectors, fuel pump (likely higher pressure), fuel lines, and a complete fuel management system.
* Engine Control Unit (ECU): You'd need a compatible ECU to control the fuel injection system. This would require extensive wiring and sensor integration.
* Sensors: A multitude of sensors are necessary for a fuel-injected system to function properly (e.g., oxygen sensor, throttle position sensor, crankshaft position sensor, manifold absolute pressure sensor, etc.). These sensors wouldn't exist on the carburetor-equipped engine and their installation points might not even be available.
* Wiring Harness: The existing wiring harness wouldn't be compatible. You'd need a complete fuel-injected wiring harness, or extensive rewiring of the existing one. This is a massive undertaking.
* Intake Manifold: The intake manifold would likely need to be replaced with one designed for fuel injection.
* Programming: The ECU would likely need to be programmed or tuned to work correctly with your specific engine. This usually requires specialized equipment and knowledge.
In short: While technically *possible*, converting a carbureted 3.9L Dodge Dakota to fuel injection is a monumental task requiring significant mechanical and electrical expertise, specialized tools, and considerable time and expense. It's generally far more cost-effective and simpler to either rebuild/replace the existing carburetor system or find a used engine that already has fuel injection. The cost and time involved in a conversion would likely exceed the value of the truck.
