Mine came on once about a month ago and haven't seen it since. There is a TSB out there for 08's that might be related...
17. POWER TRAINRIVELINEIFFERENTIAL UNIT Bulletin Number: 08-04-21-003
Bulletin Date: APR 2008
Vehicle: 2008 GMC Acadia
SERVICE ALL WHEEL DRIVE AND/OR ALL WHEEL DRIVE OFF MESSAGE DISPLAYED IN DIC, DTC C0394 SET (INSPECT REAR DIFFERENTIAL WIRING HARNESS AND REPAIR, IF NECESSARY). ( NHTSA ITEM NUMBER - 10025349 )