The corpses were tied with wire and rope material from the overpass, which connects Michoacan to the country's most populous region, the central State of Mexico, authorities said.
"For the moment, we don't know the identity of the dead or the motive of the crime, though we presume a criminal organization is behind this because of its characteristics," said Magdalena Guzman, spokeswoman for the Michoacan state prosecutor's office.
Drug cartels have often hung bodies of their victims from bridges to send a message to their enemies.
The cartel's leaders have been detained and the vigilantes have been folded into an government-recognized "rural defense" force, but violence continues in the state.