Shield Grunts are an advanced version of the Orc Grunts, but they have the savagery and nobility of the Orcs.
The Orc Grunts are vulnerable to ranged weapons from Human attacks, they needed protection to counteract this, and they have now carried their shields for their armament alongside with their battleaxes.
The Shield Grunts are still seen in Alterac Pass map in Heroes of the Storm as melee minions for the Horde side.
Update: Added defend animation.