I am pretty sure you can get this to work on a server with only 3.5 installed, but if nothing else you can always install MS Ajax 1.0 to install the 1.0.61025 version of System.Web.Extensions.dll.
Before you do that, though, make sure you have the following section in web.config:
Code:<runtime>
<assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1" appliesTo="v2.0.50727"><dependentAssembly>
<assemblyIdentity name="System.Web.Extensions" publicKeyToken="31bf3856ad364e35"/>
<bindingRedirect oldVersion="1.0.0.0-1.1.0.0" newVersion="3.5.0.0"/>
</dependentAssembly>
<dependentAssembly>
<assemblyIdentity name="System.Web.Extensions.Design" publicKeyToken="31bf3856ad364e35"/>
<bindingRedirect oldVersion="1.0.0.0-1.1.0.0" newVersion="3.5.0.0"/>
</dependentAssembly>
</assemblyBinding>
</runtime>
Roger Martin
Creator and Lead Developer of Gallery Server Pro