tabledrag: React on drag event
Got the answer for you! Check out the core tabledrag.js lines 862, 869, 1150, 1160. These are all functions that you can overwrite to react to tabledrag events! Here's a code example I wrote for you:

Drupal.tableDrag.prototype.row.prototype.onSwap = function (swappedRow) {
      (function ($) { // Important as this allows $ for jQuery.
        alert('React to tabledrag!');
        return NULL;
      }(jQuery));
    }