A couple of more findings:

(To repeat this is an upgrade from 5.6.2 directly to 7.3.4)

Testing against a 7.1 install, when clicking the upload button under File Manager, the popup activates immediately. On the upgraded 7.3.4 site there is a post back to the page and then the page locks. So is there a setting missing that forces DNN to do the popup instead of the post back? I've checked the Host :: Site Management :: I}Usability Settings :: User Popups is checked.

Additionally, when I try to run the Admin Toolbar :: Tools :: Upload File, it fails. Digging into the code shows that the FileUpload object is not built correctly. Walking the code during page build, shows that the "options" object passed to the dnn.createFileUpload function has all the keys first letter Capitalized, while on the 7.1.x install these keys are start lower case. This causes the instance creation to not complete because it can't find options.clientId (when the actual key is options.ClientId).

upgraded site options object:

1. options: Object
   1. ClientId: "fileUploader"
   2. Extensions: Array[0]
   3. FolderPath: null
   4. FolderPicker: Object
   5. Height: 630
   6. MaxFileSize: 8388608
   7. MaxFiles: 0
   8. ModuleId: ""
   9. Parameters: Object
   10. ParentClientId: null
   11. Resources: Object
   12. ShowOnStartup: false
   13. Width: 780
   14. __proto__: Object

7.1.x non upgraded site:

1. options: Object
   1. clientId: "fileUploader"
   2. extensions: Array[0]
   3. folderPath: null
   4. folderPicker: Object
   5. height: 630
   6. maxFileSize: 29360128
   7. maxFiles: 0
   8. moduleId: ""
   9. parameters: Object
   10. parentClientId: null
   11. resources: Object
   12. showOnStartup: false
   13. width: 780
   14. __proto__: Object

Any idea where the options Object is created so I can investigate further?

dnn.createFileUpload = function (options) {

    $(document).ready(function () {

        var instance;

        if (options.parentClientId) {

            instance = new dnn.FileUploadPanel(options);

            var $parent = $("#" + options.parentClientId);

            if ($parent.length !== 0) {

                $parent.append(instance.$element);

            }

        }

        else {

            instance = new dnn.FileUploadDialog(options);

        }

        if (options.clientId) {

            dnn[options.clientId] = instance;

        }

    });

Just to be complete: The dotNet 2.0 version of NewtonSoft.Json was installed by a 3rd party module during it's upgrade process (done after the DNN upgrade). The DNN 7.3.4 upgrade package contains only the dotNet 4.0 NewtonSoft.Json DLL.