I tried this under a similar situation on my iPhone 4 and can not get the backup to take. Long story short: My friend backed it up on his computer, jailbroke it, updated to the latest Os, and tried uploading the backup but it wouldn't take. The thought was that it needed to be done on my computer/iTunes. He gave me the backup file and I tried multiple ways but could not get iTunes to recognize the backupfile; it always went to the last file before that. I performed a factory default restore and that worked fine i.e. no errors. I tried restoring with that same file and still would not take. My friend also tried on his computer but it still would not take. Any help would be greatly appreciated.
Thanks