$JusRes=array(
"Jus 1"=>"Resp 1",
"Jus 2"=>"Resp 2",
"Jus 3"=>"Resp 3"
);

<script type='text/javascript'>
function mudaRes(indice){
    var just=document.getElementById('Just');
    var selectedOption = just.options[just.selectedIndex];
    var respValue = selectedOption.getAttribute('Resp');
    var resp=document.getElementById('DadosTab');
    var Re=document.getElementById('Res');
    Re.value = respValue;
    resp.rows[indice+1].cells[4].innerHTML = respValue;
}

</script>

<table id='DadosTab'>
  <tr>
    <th>Nome</th>
    <th>Sobrenome</th>
    <th>Idade</th>
    <th>Just</th>
    <th>Resp</th>
  </tr>";
echo "<form action='' method='POST'>";
$indtd=0;
$indtr=0;
foreach ($Dados as $chave=>$valor){
    echo "<tr id='".$indtr."'>";
    //nome
    echo "<td align='center'><input type='hidden' name='nome[]' value='".$Dados[$indtr][0]."' />".$Dados[$indtr][0]."</td>";
    $indtd++;
    //sobrenome
    echo "<td align='center'><input type='hidden' name='sobrenome[]' value='".$Dados[$indtr][1]."' />".$Dados[$indtr][1]."</td>";
    $indtd++;
    //idade
    echo "<td align='center'><input type='text' name='idade[]' value='".$Dados[$indtr][2]."' /></td>";
    $indtd++;
    //echo "<td align='center'>".$Dados[$ind][2]."</td>";
    echo "<td><select id='Just' name='Just[]' style='width: 220px' onchange=mudaRes($indtr)>";
    $indtd++;
        echo "<OPTION>Tipo de Justificacao</OPTION>";
        foreach ($JusRes as $Jus=>$Resp){
            echo "<OPTION id='".$indtr."' Resp='".$Resp."' value='".$Jus."'>".$Jus."</OPTION>";
        }
    echo "</select></td>";
    echo "<td align='center'><input id='Res' type='hidden' name='Res[]' /></td>";
    $indtd++;
    echo "</tr>";
    $indtr++;
}
echo "<tr><th align='left' colspan='5'><input type='submit' value='Send' name='Send'></th></tr>";
echo "</form>";

var Re=document.getElementById('Res');

↓
echo "<td><select id='Just$indtr' name='Just[]' style='width: 220px' onchange='mudaRes(this, $indtr)'>";

↓
echo "<td align='center'><input id='Res$indtr' type='hidden' name='Res[]' /></td>";

onchange='mudaRes(this, $indtr)'

↓      ↓
echo "<td align='center'><span></span><input id='Res$indtr' type='hidden' name='Res[]' /></td>";

function mudaRes(just, indice){...

var resp_txt = resp.rows[indice+1].cells[4].querySelector("span");
resp_txt.innerHTML = respValue;

function mudaRes(just, indice){
    var selectedOption = just.options[just.selectedIndex];
    var respValue = selectedOption.getAttribute('Resp');
    var resp=document.getElementById('DadosTab');
    var Re=document.getElementById('Res'+indice);
    Re.value = respValue;
    var resp_txt = resp.rows[indice+1].cells[4].querySelector("span");
    resp_txt.innerHTML = respValue;
}