The people taken by the shadows have Ascended to a Higher Plane of Existence.
At the end of the movie, James sees the silhouettes of Luke, Rosemary, and Paul among the shadow creatures. This means that all the shadow creatures could be the souls of everyone who vanished, and who are just trying to welcome the remainder to their final destination. However, they're going at it in a Jacob's Ladder sort of way, scaring the daylights out of the handful to are afraid to move on for whatever reason. The reason why the truck still worked while every other vehicle didn't.
There might be a reason why the truck is the only vehicle that still works while other vehicles do not. Every other vehicle that is seen are all made after 1990 while the truck is a 1985 Chevy C-20. In the 1990s, on-board computer systems were being used and slowly becoming common. Mostly, they were used to maintain ignition systems. But now, there are more vehicles on the road that utilize the computer systems than there are that do not have them. The Chevy is from 1985, which would means it was made before it became a standard. It could be because of the fact that the truck is more mechanical in it's systems that allows for it to continue to work in comparison to every other vehicle, which had computer systems running one or more systems in the vehicle.