Can't disable Telerik dropdown list with JQuery

[Rick]

I'm trying to use JQuery to enable a Telerik RadComboBox based on the selection of another Telerik RadComboBox.

Use case: User comes to page, ddlRequired is enabled, ddlListItems is disabled. If user selects "Required" then I want ddlListItems to then be enabled. Conversely, if "Optional" is selected, I want to disable ddlListItems.

<telerik:RadComboBox ID="ddlRequired" runat="server" Width="80px" ShowDropDownOnTextboxClick="true" EnableEmbeddedSkins="false" Skin="Classic"  OnClientSelectedIndexChanged="OnSelectedIndexChanged">
                    <Items>
                        <telerik:RadComboBoxItem Text="Required" Value="1" />
                        <telerik:RadComboBoxItem Text="Optional" Value="0" />
                    </Items>
                </telerik:RadComboBox>

                <telerik:RadComboBox ID="ddlListItems" runat="server" Width="200px" ShowDropDownOnTextboxClick="true" EnableEmbeddedSkins="false" Skin="Classic" Enabled="false">
                    <ItemTemplate>
                        <asp:TextBox ID="txtBoxQ1" runat="server" Width="160"/>
                    </ItemTemplate>
                    <Items>
                        <telerik:RadComboBoxItem />
                        <telerik:RadComboBoxItem />
                    </Items>
                </telerik:RadComboBox>

Here's where I've left off after trying several approaches. I realize it's not complete.

function OnSelectedIndexChanged()
{
    var ddl = $find("<%=ddlListItemsQ1.ClientID%>");
    ddl.set_enabled(false);
}

[Jayesh Goyani]

Hello,

<telerik:RadCodeBlock ID="RadCodeBlock1" runat="server">
       <script type="text/javascript">
           function onSelectedIndexChanged(sender, eventArgs) {
               var selectedItem = eventArgs.get_item();
               var ddlListItems = $find("<%= ddlListItems.ClientID %>")
               if (selectedItem.get_value() == "1") {
                   ddlListItems.disable();
               }
               else {
                   ddlListItems.enable();
               }
           }
       </script>
   </telerik:RadCodeBlock>

<telerik:RadComboBox ID="ddlRequired" runat="server" Width="80px" OnClientSelectedIndexChanged="onSelectedIndexChanged">
            <Items>
                <telerik:RadComboBoxItem Text="Required" Value="1" Selected="true" />
                <telerik:RadComboBoxItem Text="Optional" Value="0" />
            </Items>
        </telerik:RadComboBox>
        <telerik:RadComboBox ID="ddlListItems" runat="server" Width="200px" Enabled="false">
            <Items>
                <telerik:RadComboBoxItem Text="1111" Value="1111"  />
                <telerik:RadComboBoxItem Text="2222" Value="2222" />
            </Items>
        </telerik:RadComboBox>

let me know if any concern.

Thanks,
Jayesh Goyani