Exactly the same thing has just happened to me, 2008 6 speed auto grand voyager V (RT). All forward gears are fine but very little or no reverse gear. Dropped the transmission oil and filter to replace them both (cheapest option to try), but alas, still the same. Found some lovely little teeth in the transmission sump as well. Good job I work in a garage, looks like it's gonna be a gearbox out job ha ha