A quick and dirty client side script to check directory depth, build a relative path and add a JQuery script element. After the first self-executing anonymous function a second fires with a slight delay (if JQuery isn't yet initialized) to verify that JQuery is loaded & ready to rock.

jquery_loader.js

(function() {

    // Pt. I - Put this near the top of the head to append JQuery (In case you need doc.ready or whatnot).

    var pageDepth = 0, relPath;

    for (var path = location.pathname, len = path.length, c = 0; c < len; ++c)
       if (path[c] === '/')
           pageDepth += 1;

    if (pageDepth > 1)
       for (var i = 0; i < pageDepth; relPath += '../', ++i);
    else
       relPath = './';

    var headH = document.getElementsByTagName('head')[0];

    // JQuery
    var jqueryTag = document.createElement('script');
    jqueryTag.src = relPath + 'js/lib/jquery.min.js';

    headH.appendChild(jqueryTag);

})();

(function(window) {
    
    // Pt. II - Put this near the bottom of your page (or wherever you like) to do the magic.

    var jqueryChk = function() {
        
        this.pass = this.pass || 0;
        
        // Check if JQuery is undefined and if it is recheck 
        // 499 more times before the recursive call bails.
        if (typeof $ === 'undefined' && this.pass++ < 500) {
            setTimeout(jqueryChk, 10);
        } else {
            // Do JQuery ninja stuff here...
        }

    };
 
    jqueryChk();

})(window);