Once all of the grease is out of the joint it will start clicking and at some point after that come apart.
You can replace just the boot but since the axle more than likely has to be removed to do it it makes more sense just to replace the whole half shaft since you don't know the condition of the joint and the half shaft is only $80 so it's not a huge expense. I'm not sure on the Liberty's but I know I can do a half shaft on my Dakota in about a half hour.
Hers - 2005 Jeep Grand Cherokee Laredo 3.7 4x4
Mine - 2003 Jeep Wrangler X