provider: SQL Network Interfaces, error: 26 - Error Locating

Boa Noite !!

Sou iniciante em ASP e estou usando Visual Studio 2005 e SQL, estou fazendo uma aplicação basica, localmente esta funcionando normal, mas quando subo para o servidor WEB, esta gerando o seguinte erro.

Server Error in ´/´ Application.
--------------------------------------------------------------------------------

A network-related or instance-specific error occurred while establishing a connection to SQL Server. The server was not found or was not accessible. Verify that the instance name is correct and that SQL Server is configured to allow remote connections. (provider: SQL Network Interfaces, error: 26 - Error Locating Server/Instance Specified)

Eu sei q é referente a configuração do banco de dados, mas não sei como fazer, segue o meu web.config abaixo:

<?xml version=´1.0´?>
<!--
Note: As an alternative to hand editing this file you can use the
web admin tool to configure settings for your application. Use
the Website->Asp.Net Configuration option in Visual Studio.
A full list of settings and comments can be found in
machine.config.comments usually located in
\Windows\Microsoft.Net\Framework\v2.x\Config
-->
<configuration xmlns=´http://schemas.microsoft.com/.NetConfiguration/v2.0´>
<appSettings/>
<connectionStrings>
<add name=´ConnectionString´ connectionString=´Data Source=.\SQLEXPRESS;AttachDbFilename=/grupodomenic/web/App_data/pdunit.MDF;Integrated Security=True;User Instance=True´
providerName=´System.Data.SqlClient´ />
</connectionStrings>
<system.web>
<!--
Set compilation debug=´true´ to insert debugging
symbols into the compiled page. Because this
affects performance, set this value to true only
during development.
-->
<compilation debug=´true´/>
<!--
The <authentication> section enables configuration
of the security authentication mode used by
ASP.NET to identify an incoming user.
-->
<authentication mode=´Windows´/>
<!--
The <customErrors> section enables configuration
of what to do if/when an unhandled error occurs
during the execution of a request. Specifically,
it enables developers to configure html error pages
to be displayed in place of a error stack trace.


<error statusCode=´403´ redirect=´NoAccess.htm´ />
<error statusCode=´404´ redirect=´FileNotFound.htm´ />
</customErrors>
-->
<customErrors mode=´Off´/>

</system.web>
</configuration>


Se alguem quiser dar uma olhada no erro vai a URL = www.grupodomenico.com.br/credenciados.aspx

Abraços e obrigado a quem puder ajudar.