... vedi se lo volevi in questo modo
Codice PHP:

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
"http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<title>Documento senza titolo</title>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
<script language="javascript">
function verificaStato(){
    if (document.getElementById('id_1').checked) {
        document.getElementById('id_input').disabled = false;
    }else if(document.getElementById('id_2').checked) {
        document.getElementById('id_input').disabled = true;
    } 
}
</script>
</head>
<body>
<form name="lista" method="post" action="">


<input name="modItem" type="radio" id="id_1" value="1" onChange="verificaStato()">1</p>


<input name="modItem" type="radio" id="id_2"value="2" onChange="verificaStato()">2</p>
</form>
<form name="azioni" method="post" action="">
<input disabled type="submit" id="id_input"name="modifica" value="Invia">
</form>


 </p>
</body>
</html>