The front passenger door on my '98 Cherokee sport is perpetually stuck in the unlocked position. It is equipped with power locks, and I can hear the actuator click, but it doesn't engage the lock; it won't even lock manually (it won't budge at all). The rear passenger door also doesn't respond to the power lock command, but can be locked manually.

Any ideas?

Have you pulled the door panels and checked the linkage? That would be my first suggestion. The fact that they feel seized means that either the linkage arms have bound somewhere or the lock cylinder has frozen up.