This is usually the vacuum operated hubs either failing to operate or the solenoid failing to say they have operated. Its a technically simply but access wise a pain in the neck job.
Check the solenoids first - they are under the relay/fuse box by the battery - clean them out., they have little foam "air filters" in them that clog. Then check that the vacuum pipes are all connected and there are no splits in the system
Finally if none of this works then strip and clean the hubs themselves.
Martin