While he's not explicitly called a jackal in the chapter, the narrator uses imagery and language that connects him to this animal. For example, the narrator describes him as "watching the wine-shop door" and "scenting out the prey." This imagery is meant to suggest that he is lurking and waiting for an opportunity to pounce.
Monsieur Defarge is not just a passive observer of the suffering of the people. He actively encourages their anger and resentment, feeding it with his own personal vendetta against the aristocracy. He represents a dangerous force in the revolution, a figure who is willing to use any means necessary to achieve his ends.