Assuming the username you are logged into is `bas`:
mklink /j "C:\\Users\\bas\\AppData\\Roaming\\Apple Computer\\MobileSync" "D:\\ITUNES"
And there you have it. Backup will now be in D:\ITUNES.
mklink /j "C:\\Users\\bas\\AppData\\Roaming\\Apple Computer\\MobileSync" "D:\\ITUNES"
And there you have it. Backup will now be in D:\ITUNES.