Yeah, just the bulb.. You won't be needing the whole new assembly because the assembly wouldn't even come with a new light bulb.
I've yet to change mine on the Liberty, but it's typically secured to the back of the headlight housing with a plastic collar that you unscrew or unlock and then pull out. Carefully use a small screw driver to release the latch between the wireharness and the bulb housing and pop off the burned out bulb. Then just slide the new one in until it clicks into place. Reinstall and secure with the collar.
Note: Try to avoid putting your fingers on the glass part of the replacement bulb. The oil residue left on the glass can heat up and shatter your brand new hallogen bulb. Thus resulting in having to do the whole process over again.
Double note: The reason the high beam will work and the low beam wont is because there are actually two seperate fillaments in the bulb. Since the low beams are on more - it always burns out first (unless there is a defect with that particular bulb)
Good luck / enjoy!