Body shops aren't exactly the best for mechanical issues.
You didn't mention whether this happened immediately or if it's recent.
FWIW - tire/wheel assembly balance issues usually show up at the speed you mention, and I wouldn't be surprised if the Body shop was unable to check this themselves for lack of tire balancing equipment.
If it happened recently, you may have lost a wheel weight. Doesn't take much to get a violent shake, some Ice on a wheel one winter made one of mine shake so badly control was difficult 'till I slowed down.
Wiggle the wheels to make sure nothing is obviously loose then get it checked out.
As Elizabeth said, second opinion is badly needed - "18's" don't CAUSE these problems.
"Rubber Side Down"