If there is still power (12 volts) going to the actuator (electromagnet), it won't disengage. Unplug the power from the differential. If that solves your problem, then you have voltage at the connector when you shouldn't. Not sure what controls that (BCM maybe?)
If you unplug the power and it is still locked, then you have an issue with the locking mechanism inside of the differential. Also, if you remove the indicator switch from the differential, you cannot replace it without taking the carrier out of the housing. Don't remove this switch.
If the mechanism is just stuck for some reason, you can probably move the actuating plate away from the differential (remove rear cover to do this). Draining is required :-(