Overview
If you are wondering that since you liberated all the areas and there are no pickup truck now, dont worry this will help you complete the live event!
Pickup Blowup
1) Go to Faiths region near Faiths Gate as marked in the map there you will find a spawn of Pickup truck
2) The exact location of pickup truck is marked and you can see it in this image.
3) Now blow it up, after the blowup go to the bliss field at end of the road as marked on the map and come back to the pickup truck spawn.
4) You will see as the pickup truck has respawned.
5) Do this again and again until you complete the required achievement.
Thanks to DANNYonPC for Tips and Tricks Video with this and more spawn locations about the Live Event which you can watch here –