the easiest way i would think is to do something like this when you're moving it:

var gridSpacing = 2.0; // put this at the top of your script

//move the object like normal, then clamp it with the code below (if you want this in the object's script, change yourObject to transform)

var position = yourObject.position;
position.x = Mathf.Round(position.x / gridSpacing) * gridSpacing;
position.z = Mathf.Round(position.z / gridSpacing) * gridSpacing;
yourObject.position = position;