JSON object serialization/deserialization using WCF service without ScriptManager

I've got a 3.5 WCF service in IIS that is communicating with the client via an AJAX call.  Because of design considerations, I can't use the ScriptManager functionality (which would have made this a no-brainer).  My problem is I need to transmit a JSON object to the WCF method.
I can transmit simple data types (int, string, etc), but when I try to transmit a more complex JSON object, the call fails.  For some reason, it's not able to convert { "Id" : "52a2aa9e-64ff-4764-8798-9c19295a0bca",  "Name" : "User Name" } into my "JsonAssignment" object which has two properties "Id" and "Name"
Here is my client code:
$.ajax({
                type: "GET",
                url: "WebServices/AiccWrapp ...