Q: lost all of my audiobooks when I upgraded iOS

How did you do the update ? If directly on the iPad then you shouldn't have lost any content from it. Or did you restore to a backup after updating ? Audiobooks (and tones) are one-time only downloads, they won't be redownloaded by an iCloud restore, and an iTunes restore will look for them in the Books/Audiobooks section of your computer's iTunes library to copy them back.

If you updated via your computer's iTunes then did you copy them over to your computer's iTunes library via File > Devices > Transfer Purchases prior to starting the update ? When you update via a computer's iTunes the process will take backup, download the new iOS version, install the new iOS version and restore to the backup - with the restore looking for your content (audiobooks, apps, ibooks etc) in that iTunes library, they aren't included in the actual backup.

If you haven't got copies of them in your computer's iTunes library nor on your backup of your downloads/library so that you can sync them back, then if they are still in the store and you haven't moved countries since buying them, then you could try contacting iTunes Support and see if they will grant you a redownload : http://www.apple.com/support/itunes/contact/