var request = null;

function createRequest() {
    if (request != null)
        return;
    try {
        request = new XMLHttpRequest();
    } catch (trymicrosoft) {
        try {
            request = new ActiveXObject("Msxml2.XMLHTTP");
        } catch (othermicrosoft) {
            try {
                request = new ActiveXObject("Microsoft.XMLHTTP");
            } catch (failed) {
                request = null;
            }
        }
    }
    if (request == null)
        alert(" :-( ___ Error creating request object! ");

}

function ajax(ins) {
    createRequest();
    var url = "index.php?inst=" + ins;
    request.onreadystatechange = update;
    request.open("GET", url, true);  // ??? Собственно вопрос <- если заменить на POST будет правильно работать???
    request.send(null);
}

function update() {
    if (request.readyState ==  4 ) {
       document.getElementById('result').innerHTML = request.responseText;
              request = null;
    }
}

<form method="post">
<input type="hidden" name="X1" value=" много букав" />
<input type="hidden" name="X2" value=" много букав" />
</form>
 
<script type="text/javascript">
var X1 = "znachenie";
var f = document.forms[ 0 ];
f.X1.value = X1;
f.X2.value = X2;
f.submit();
</script>