Tentou colocar no displayFields e usar o setEnabled, dessa maneira:

form.setEnabled("txtCampo", (atividade == 5 ));

Guilherme, são muitos campos a bloquear? São campos pré-definidos para a etapa em questão? Se sim, sugiro encapsular dentro de uma div específica para cada etapa e bloquea-las de acordo com sua necessidade. Faço isso e funciona perfeitamente.

enableContainer($("#" + nomeEtapa)[0], true); bloqueia todos os campos dentro da div enableContainer($("#" + nomeEtapa)[0], false); desbloqueia todos os campos dentro da div

function disableField($el, disabled){
    if(disabled){
        $("#" + $el.attr('id') + "_d").hide();
        $el.show();
    }
    else{
        ($("#" + $el.attr("id") + "_d").length > 0) ? $("#" + $el.attr("id") + "_d").show() : $el.before($el.clone().attr({"id":($el.attr("id") + "_d"),"name":($el.attr("name") + "_d")}).attr("disabled",true));
        $el.hide();
    }
}
function enableContainer($el, enabled){
    $($el).find("input[type='radio'],input[type='text'],input[type='number'],input[type='checkbox'],textarea,select,input[type='button'],img").each(function (i) {
        enableField($(this), enabled);
    });
};
function enableField($el, enabled){
    if($el.attr("type") == "text" || $el.attr("type") == "number" ){
        $el.prop("readonly",!enabled);
        //se possui botao agrupado desabilita tambem
        if(enabled == false){
            $el.parent().find('.input-group-addon').css("pointer-events", "none");
            $el.parent().find('.in    put-group-addon').children().css("opacity", 0.4);
        }
        else{
            $el.parent().find('.input-group-addon').css("pointer-events", "auto");
            $el.parent().find('.input-group-addon').children().css("opacity", 1);
        }
    }
    else if($el.prop("tagName") == "TEXTAREA"){
        $el.prop("readonly",!enabled);;
    }
    else if($el.prop("tagName") == "SELECT"){
        disableField($el, enabled);
    }
    else if($el.attr("type") == "button" || $el.prop("tagName") == "IMG"){
        $el.prop("disabled",!enabled);
        if(enabled){
            $el.css("opacity", 1);
            $el.css("filter", "");
            if($el.data("click-event") != ""){
                $el.on("click", $el.data("click-event"));
                $el.data("click-event", "");
            }
        } else {
            $el.css("opacity", 0.4);
            $el.css("filter", "alpha(opacity=40)");
            $el.data("click-event", $el.get(0).onclick);
            $el.off();
            $el.get(0).onclick = "";
        }
    }
    else if($el.attr("type") == "radio" || $el.attr("type") == "checkbox" || $el.attr("type") == undefined){
        var endWithDisabled = new RegExp(/_d$/);
        var nameOf = ($el.selector.replace("#","") != "") ? $el.selector.replace("#","") : $el.attr("name");

        $el = $("[name='" + nameOf + "']").filter(function(index, element) {
            return !endWithDisabled.test(element.id);        });

        if($el.length && $el.length > 0 && ($el.attr("type") == "radio" || $el.attr("type") == "checkbox")){
            $el.each(function(i){
                $("label[for^='"+$(this).prop("id")+"']").each(function (i) {
                    var suffix = (endWithDisabled.test($(this).prop("for"))) ? "_d" : "";
                    if(enabled){
                        $(this).prop("for", $(this).prop("for").replace(endWithDisabled,""));
                    }
                    else if(suffix == ""){
                        $(this).prop("for", $(this).prop("for")+"_d");
                    }
                });
                disableField($(this), enabled);
            });
        }
    }
}