All of my RAW photos are already stored on several external drives. The Catalog lrcat files are stored on my internal HD. There is a (1.87GB) LR2 Catalog and (3.2GB) Data file and a (1.61 GB) LR5 Catalog and (115.27 GB) DATA file. There are also 18 separate Temporary folders and one Backups folder.
First, move the backups folder to an external drive. If that doesn't help, see what else you don't need (not necessarily Lightroom files). Lastly, move the catalog files and associated previews to the external drives (but be aware they may work more slowly on an external drive).
By the way, do you really need both a LR2 and LR5 catalog? Are you using both? Do they catalog different photos?
I only use Lightroom 5 now and I am pretty sure that the LR2 and LR5 are the same but I haven't checked. Where are the preview photos kept? I have moved everything off my computer that I can already so unfortunatly the LR files have to go.