Problem with z-index and display

I'm building a site here and on the home page I have two tabs off to the right side that use javascript to switch the display property of two divs. This system all works fine and dandy in everything but IE of course... IE doesn't seem to recognize the z-index that each of the items is on and the main picture and text get shifted down when you click on one of the tabs. Can anyone help me resolve this problem?

Here's my CSS:

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