If the maincontent area exceeds 940px, IE will decide not to show the navigation (very odd, but it's IE). A quick glance at http://health.unl.edu/
shows that your maincontent does indeed exceed 940px (notice the small gutter on the right side).
The likely cause (though it might not be the only) are the two zenboxes inside the two_col div on the bottom left. Remove one of them.