FileBrowser app will let you browse Dropbox and even shared folders on your computers, but it does not remember the play position for audiobooks, which is annoying.
You may be able to transfer the files to the iPad using IFunBox on your computer, but I have not done it this way so don't know how well it works.
Edit: this came up in an App Store search.
https://itunes.apple.com/us/app/clou...573192227?mt=8