"ID=951357852456&FNAME=Jaime&LNAME=Lopez"

{ "ID":"951357852456","FNAME":"Jaime","LNAME":"Lopez" }

var dict = HttpUtility.ParseQueryString("ID=951357852456&FNAME=Jaime&LNAME=Lopez");
var json = new JavaScriptSerializer().Serialize(
                    dict.AllKeys.ToDictionary(k => k,k => dict[k])
           );