Okay, i found the crux of my issue - whenever a postback occurs from something other than the dropdown list, the "selected index" of the dropdown is reset to -1 (basically "not selected"). However, the visual aspect of it still shows the item I selected... which makes no sense if the index is set back to -1... which then also makes it impossible to "reset" it via javascript, as nothing is actually selected...
So, unless someone says otherwise, it looks like I have to store the selected value in a hidden text box somewhere and then reference that in my code-behind in postback, then reset the value from there, as that works just fine...