95 4 dr. Chevy Lumina Door Locks 112K miles

My grandpa's 1995 4 dr. Chevy Lumina has 112K miles. Just last week a particular feature stopped working. When you put the car in gear and began driving, the doors would lock. Also, when you stopped and put the car in park, the doors would unlock. It is no longer doing this. What has happened and how do we fix? The door lock switch on the doors still work, as well as, the factory remote for keyless entry.