I am not aware of any freeware that will repartition on the fly, however, I can highly recommend Drive Genius by ProSoft Engineering to do the job.
A few weeks ago OWC had a deal where it was only $USD 59, which is a good deal.
Consider that this app does a lot more than repartition, so you're getting a lot of bang for your buck, especially if it helps you recover a damaged or wonky machine in the future.
Hard to put a value on that...
MD