It happened at 10:00 a.m. Tuesday in the unit block of Sandlewood Drive.
The worker parked his vehicle and got out, but it began to drift backwards.
The 21-year-old tried to stop the truck but became pinned.
Rescuers got him out from under the vehicle and rushed him to Christiana Hospital.
Witnesses say the truck was not in park but police say the investigation is ongoing.