Not sure if you still need help with this, trying to clear up some older questions.
If I understand correctly, on you site the “post” post type has a custom field on it that is named “drivers” and this field is a relationship field.
If this is true then what you have should work. The only thing I can come up with looking at your code is that you’re either querying the wrong post type or that the field name is not “drivers”
