<?php
function cat_menu() {
    $items['ajax_node/%'] = array(
        'page callback' => 'cat_print_json',
        'page arguments' => array(1),
        'type' => MENU_CALLBACK,
        'access callback' => true
    );

    return $items;
}

/*
 * JSON Output of node
 */
function cat_print_json($nid) {
    $node = node_load($nid);
    $data['body'] = $node->body['und'][0]['safe_value'];

    print drupal_json_encode($data);
    exit();
}
?>