Hi Tux,
On the load balancing environment, you must set a UNC path for uploader as the TempDirectory, and must use the same location.
UNC path: http://www.cutesoft.net/developer+guide/scr/UNC.htm
TempDirectory: http://www.ajaxuploader.com/document/scr/BeforeYouBegin.htm, the first section
On both servers, the uploader must use the same Temp Directory(mean that, if uploader which on server "A" use '192.168.1.123/photos' as the TempDirectory, the uploader on server "B" must use this folder as the TempDirectory too).
Regards,
Ken