As an alternative, check this out:
They are height-adjustable, too. You hook one of those puppies over your tire, and it's easy to throw a bike on the roof. Also convenient to have one of those around anyway, especially if you ever put anything else on the roof. $15 or less at your local auto-parts store.
Make sure to get Thule bigmouth racks:
(from the desc: Easy to mount, convenient rack level bike clamping feature. Worry free automatic self-adjusting jaws for perfect bike clamping every time. Dual locking design locks bike to carrier and carrier to rack.) You can throw the bike up there and clamp it in in a matter of seconds, and you don't have to reach all the way up to the frame to clamp it in. Theft won't be an issue, they'd have to steal the whole rack off the roof (including the factory bars). It's so high up, it'll never get hit by a passerby. And the only in-cab space taken up is for the tire step.
If you really want to get an interior rack, I suggest looking into pickup bed mounts and modifying them. I think you can probably get a fork-mount in there somewhere without too much hassle. The bike will fill up the whole back of the liberty, though, and I'd actually be surprised if it will fit standing up in one of those fork mounts. You'd be best off to fold the seats down and just lay the bike on its side.