This is not uncommon with helical gears (diff and TC), and the reason for backing up. Unlocking the hubs relieves the load from the drivetrain, allowing the gears to disengage. You can do this by unlocking only one hub, or backing up, or driving a distance with some on/off loading. Moving the TC shifter is passive, meaning it only "suggests" the desired gear range (2H/4H). The compulsory "back up" is specifically related to the auto hubs, which are also loaded helical gears.