Far Cry 4

If you have completed all outposts and none are spawning just hit start>Campaign settings and reset all outposts. They will spawn almost instantly.

You can spawn one everytime by fast traveling to the airport! After delivering the cargo, fast travel accross the map then back to the airport.

The airport is top left of the map looks like a long strip when zoomed in.

I did this after liberating all outpost however I did have 1 fortress unliberated closest to the airport.

You must deliver the truck destroying doesnt count.

You recieve the achievement after delivering your third truck.

Throw me a like if I helped you.

After completing the Prologue, and liberating the first Outpost, Royal Cargo trucks will begin to appear throughout the world (look for the blue truck icon) and will be available to steal. When one appears you can either steal it and return it to the nearest Outpost, or destroy it and receive a smaller reward. Hijack 3 of these trucks and return them to unlock this achievement! To hijack a truck just shoot the driver or shoot the truck until the driver gets out, shoot him and drive off. Don't worry if you miss the truck, as the truck seems to come back to your position after about 30 seconds. Good luck!

The other solutions do a great job, but none of them really tell you where to look for the trucks. In order to get a royal cargo truck to spawn, you need to be in enemy territory, which is marked on your map with a red haze. These areas are found near un-liberated outposts and fortresses. Simply drive around in enemy territory until a blue truck marker appears on your map. To hijack the truck, kill the driver or do enough damage to the truck to get the driver to pull over. You'll need a vehicle in order to keep up with the truck. Afterward, you'll have to drive it back to a liberated outpost for credit. Do this three times to unlock the achievement.

It's a good idea to try to get the trucks to spawn on the edge of enemy territory as it's less of a drive back to your outpost and you have less of a chance of spawning enemies while you're trying to deliver the vehicle.

If you've already liberated all the outposts and fortresses, you can enter the pause menu, select "Gameplay (Campaign)," scroll to the bottom and click "Reset Outposts." This will reset the outposts you have liberated and create enemy territory all over the map, which will allow you to spawn cargo trucks again. Happy hunting!

This is for if you have taken over all outposts and fortresses EXCEPT FOR RAJGAD GULAG if you haven't taken over Min's fortress here are the steps.

Step 1: Go to the entrance of it (beating the game to have the front door blown off is highly recommended.)

Step 2: Kill a few people and get detected

Step 3: Save the game *in the fortress* and then let the enemies kill you. (getting as far as you can into the fortress while still being able to save would be recommended just to make sure it counts.)

You should respawn to the left of the entrance. (If you spawn to the right trucks don't spawn nearly as often, to prevent spawning to the right, drive to the fortress from the left side.)

Step 4: Wait a minute and if a royal cargo truck doesn't come, repeat steps 1-3. (it usually takes 2-3 times for a truck to come.)

Step 5: When a Royal Cargo truck comes, hijack it and bring it to the nearest outpost (the nearest outpost is the Namboche Monastery, so liberating it before-hand makes things a little easier.)

If all else fails, keep driving back and forth between the Monastery and the fortress and a truck will appear about as fast as the first method.

This could also help out with the "Defender" achievement if you don't have it yet, as going back and forth between the outpost and fortress spawned 2 retaliation parties for me.