is_user_logged_in() throwing undefined function error

It’s failing because WordPress isn’t loaded. Instead of making AJAX requests directly to PHP files in your theme, make them to a REST API endpoint.

For example:

add_action( 'rest_api_init', function () {
        register_rest_route( 'petebolduc/v1', '/test/', array(
                'callback' => 'petebolduc_ajax'
        ) );
} );
function petebolduc( $parameters ) {
    $foo = $parameters['foo'];
    return "foo is " . $foo;
}

With that code, visiting: example.com/wp-json/petebolduc/v1/test?foo=bar

gives:

"foo is bar"