vibration in first gear only automatic transmission

what would cause vibration in first gear only on an automatic 1996 mirage

This could be a few things. It could be the transmission, but it could be that under the load of getting the car moving in first gear, you are getting a vibration from the drivetrain. You may want to take this to a shop and take a technician for a drive to demonstrate this for the them and see what they think.