I would realy like to see the HTML for the form. But for now my guess would be (assuming name of your form is "form"):
instead of
document.form.tabbedPanel1.mapDates.lstDateRange.disabled
you should use
document.form.lstDateRange.disabled
instead of
document.form.mapDates.lstDateRange
use
document.form.lstDateRange
instead of
document.form.lstDateRange.length
use
document.form.lstDateRange.options.length
instead of
document.form.tabbedPanel1.mapDates.lstDateRange.options[i].selected
use
document.form.lstDateRange.options[i].selected
I probably missed something

, but try that.