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Jenelle Evans whipped out a handgun -- while her son was riding shotgun, btw -- and brandished it at a driver she says cut her off during a road rage incident.

MTV aired video of the incident on Monday night's "Teen Mom 2" and it shows Jenelle slam on her brakes suddenly, start yelling at the other driver ... and then reach for her gun under the driver's seat, and place it in her lap. Remember, her 8-year-old son, Jace, is sitting right next to her.

Jenelle stopped and got into an argument with the guy, yelling, "You were tailgating me, you dumbass!!"

According to the other driver, Jenelle followed him to his house and drove into his mailbox. He then appeared to make contact with Jenelle's vehicle, and that's when she pulled the gun.

MTV opted not to air video of her brandishing the gun during the April incident -- but it's clear they have the footage.