Hello...
i had hundreds of problems with Jquery in modx but everything was because there are some plugging that works with Mootools.
SO. just find or set all the plugging for jquery and never mix mootools and jquery in the seem project. (there are a java code to fix this but i thing that is better dont mix codes)
so usually in the settings in every plunging there are the option Jquery and Mootools, tryto find it!
and another thing is that Modx Call Jquery automatically to be used wen you are logging and working with the mannager, so everithing works perfectly. But wen you are loggout, that is the problem, some jquery thing doesn.t work properly. So, my advise is that you need to call a jquery file by your self like normally you do in html:
<script src="assets/templates/default/js/jquery-1.4.2.min.js" type="text/javascript"></script>
do after that, Yes modX works perfectly with jquery, i really love jQuery and ModX is really Amazing!!!
i hope my answer helps! and sorry for my english is not my main language!!!

see you!!!