Originally Posted by Gray Mage
One thing to keep in mind is that even True Resurrection isn't capable of bringing someone who has been turned into an undead if the undead isn't destroyed. That implies that undead animation somehow messes with the person's soul, so it's not only the person's "shell".
Actually, RAW is self-contradictory on this one.
The spell description says it can't affect undead unless they're destroyed, but the description of the undead type in the MM says that an undead targetted by true res' turns back into a living creature.
Ask your DM.