Elemental Rage (Su): While raging, all of the barbarian's melee attacks deal an additional 1d6 points of energy damage (acid, cold, electricity, or fire). The type is chosen when the barbarian begins her rage. A barbarian must have the lesser elemental rage power to select this rage power. A barbarian must be at least 8th level to select this rage power. Note that the barbarian can still use her lesser elemental rage power while using this rage power, but she must select a different energy type.
Is there any reason that one couldn't use a Shock Weapon and add an additional +1d6 Electricity to the attack? So weapon damage +2d6 Electricity?
Nope, as far as I know, they stack. I asked a similar question to James Jacobs about the feat Elemental Fist and Amulet of Mighty Fist with an elemental property. He said Elemental Fist (which adds 1d6 energy damage, more if you're a Monk of the Four Winds) and an Elemental AoMF stack so I see no reason why Elemental Rage wouldn't either.