If you have manually populated your drop-down list, then you probably can't do this (at least not in any useful and easy manner). If you are populating the drop-down list from a secondary data source, then you just grab the stored value (the ID), and you look up the display value in the secondary data source using the ID value you have.
Something like:
var disp = XDocument.GetDOM("2DS").selectSingleNode("/Root/Table/Row[@id = " + ID + "]/@display");