Hi Jon,
When you run a config save, the config running in memory is saved to the files and then wrapped up in a tar ball. If you aren't running GTM, WAM or ASM, you could probably use a Single Configuration File (SCF) to do a config diff more simply. As the name implies, the SCF contains the full LTM configuration in a single text file. The one notable exception is the SSL certs/keys.
You can create an SCF file using 'b export `/bin/hostname`.scf'. The output is written to /var/local/scf/ by default.
See SOL8453 for details:
SOL8435: Overview of the single configuration file (SCF)
http://support.f5.com/kb/en-us/solutions/public/8000/400/sol8435.html
Aaron