Hi Malkom,
What version of mojoPortal are you using?
With the latest source code we are not using YUI anywhere (and have not in quite a few versions) I know of except in one place and that is for a color picker in my add on product Event Calendar Pro. We used to use YUI tabs a lot be we changed to jQuery tabs.
You can find the mojoPortal version under Administration > System Information
I suspect you are using a very old version of mojoPortal.
Best,
Joe