I'll houserule that L'Aurel and Archon, assuming they threaten and confirm a critical, have a 50% chance of that actually applying, because of their status as favored enemies.
So, essentially, roll crits normally, and then there's a 50% chance that it will just be normal damage.
Justice's sword - Shadowseeker - lets her crit undead as part of its abilities. It's essentially a "Sun sword" from the DMG with a few extra details...the same sword she's had since Thanesport, in fact.
EDIT: This is a change in house rule. Before, I have allowed the two rangers to crit undead normally. That was too much - this still gives them some benefit to having undead as a favored enemy, but not so much that it is ridiculous.