Some thoughts - perhaps you can use them to derive a solution:
IF the drive has any valuable data, I won't recommend resizing it on the fly and creating new partition (unless you have a reliable backup of that important data somewhere else).
Don't use the case sensitive HFS+ unless you have a very good reason. See
http://www.macfixit.com/staticpages/index.php?page=2003111009264885 (and possibly other places) for some warnings.
Leopard's Disk Utility (DU) should be able to make partitions on the fly, IF there is contiguous space and the OS is not booted from that drive. If your drive has been in use for a while, there may not be 70GB (or 30GB) of contiguous space and that is most likely the problem. [There may be other variables like required GUID partition table.]
Given all that, your best option may be to copy at least the non-TM files somewhere else, repartition the disk (preferably with GUID partition table), them redo a full TM backup (and copy the non-TM files to the disk if you want them there.
If you feel more adventurous, you can defragment the TM disk (the ones that will do that I know of commercial versions only) and try creating a partition on the fly with DU -- but some of these commercial tools with do that without using the DU.