To find the correct capacity, you must check your vehicle's sticker (often located on the driver's side doorjamb) or your owner's manual. The sticker or manual will list the axle ratio (e.g., 3.55, 3.92, 4.10) which will help you find the correct capacity in a repair manual or online database specific to Dodge Ram Vans.
Without knowing the axle ratio, providing a definitive answer is impossible and could lead to incorrect oil filling, potentially damaging your differential.