1) Create a share on 1 server in the farm in the format \\servername\pathname. Set the TemporaryFolder in the web application on all servers in the farm to point to this share.
2) Give Everyone Full Control on the share.
3) Most importantly, create a separate application pool for the web application. Change the Identity on the application pool from ApplicationPoolIdentity to LocalSystem. Change web application to use the new application pool.
4) Suggestions for setting disable plug-ins or use impersonation on RadAsyncUpload control did not work, nor did adding security on the share for IIS_IUSR, Authenicated Users, or any other group.