<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.6.0/jquery.min.js"></script>

<form method="post" action="">
  <table class="formTable">
    <tr>
      <th>ご希望商品（選択2つまで）</th>
      <td>
        <ul>
          <li><input type="checkbox" name="ご希望商品[]" value="ご希望商品01">ご希望商品01</li>
          <li><input type="checkbox" name="ご希望商品[]" value="ご希望商品02">ご希望商品02</li>
          <li><input type="checkbox" name="ご希望商品[]" value="ご希望商品03">ご希望商品03</li>
          <li><input type="checkbox" name="ご希望商品[]" value="ご希望商品04">ご希望商品04</li>
        </ul>
      </td>
    </tr>
  </table>
</form>
  
$("input[type=checkbox]").click(function(){
    var $count = $("input[type=checkbox]:checked").length;
    var $not = $('input[type=checkbox]').not(':checked')

    if($count >= 2) {
        $not.attr("disabled",true);
    }else{
        $not.attr("disabled",false);
    }
});

External CSS

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External JavaScript

This Pen doesn't use any external JavaScript resources.