Disable/Enable Required Fields

I am wanting to make a few fields required after they are enabled, but it's not working well.

<script language="javascript">
function enableAdd(){
document.form1.empJobTitle.disabled=false;
document.form1.empExt.disabled=false;
document.form1.empEmail.disabled=false;
}

function disableAdd(){
document.form1.empJobTitle.disabled=true;
document.form1.empExt.disabled=true;
document.form1.empEmail.disabled=true;
}

function validateCheck(){
if(document.form1.empJobTitle.disabled=false && document.form1.empJobTitle.value=="");
alert("You must enter the employee's job title.");
return false;

if(document.form1.empExt.disabled=false && document.form1.empExt.value=="");
alert("You must enter the employee's extension.");
return false;

if(document.form1.empEmail.disabled=false && document.form1.empEmail.value=="");
alert("You must enter a valid email address for the employee.");
return false;
}

</script>