passing index from for loop to ajax callback function (JavaScript)

You could use a javascript closure:

for (var i = 0; i < arr.length; i++) {
  (function(i) {
    // do your stuff here
  })(i);
}

Or you could just use $.each:

var arr = [2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010];

$.each(arr, function(index, value) {
  $.ajaxSetup({ cache:false });
  $.getJSON("NatGeo.jsp", { ZipCode: value, Radius:   
    document.getElementById("radius").value, sensor: false },      
    function(data) { 
      DrawZip(data, value);
    }
  );
});