i have the z-index set high enough
The z-index for your menu is not quite high enough. It is currently set to 100 (in your 'slidingdoors-2.css' file) but it needs to be greater than 500 in order to be stacked on top of the gallery when a visitor hovers over a menu entry which overlaps the gallery.
the issue is when i try to select either "retail" or "residential" from the sub nav. i cannot and it clears away.
This is exactly the issue that I first saw and that my code above should fix.
As I mentioned, I tried the code in Firefox's developer's tools (setting z-index: 9999; dynamically on the #dropmenuexp.dropmenudiv_c container) and it solved the problem.
Please try adding the code to your web page and then clear your browser's cache before reloading your web page.
Be sure to add the code to the end of your web page's <head> section (so that it is not overwritten by the menu's own CSS).
If in any doubt, try adding CSS !important as follows:
<style type="text/css">
#dropmenuexp.dropmenudiv_c {
z-index: 9999 !important;
}
</style>