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You can use Dropsync to auto-sync /mnt/sdcard/abc to a folder in your Dropbox then sync that folder to /mnt/sdcard/Dropbox/abc.
I don't know of a way to sync the the two directly. The FAT32 file system used on SD cards does not support symlinks. I don't think Android allows non-root users to create symlinks anyway.
Unless there's a way for Dropsync to sync the root folder but allow me to exclude specific folders.