Guardian Stance: "Use while an adjacent friendly figure is defending. Reroll 1 or more attack or defense dice."

Does this mean i can reroll all dice if I want to or do I have to choose between rerolling attack OR defense dice. For example, if a trained rancor is defending against a stormtrooper, and I want to reroll both defense and both attack dice, can I do that? Or an I only reroll the 2 defense OR 2 attack dice?

I don't think you need to read too much into the or . You would be asking if you need to roll both if it were and .

You choose which combination of dice to reroll, then reroll them at the same time.

The chosen dice can be any combination of attack and defense dice.

However, this defender ability cannot reroll any die that has already been rerolled during the same attack (due to attacker abilities).

so lets say the adjacent friendly unit is defending and rolls 2 dice, and the attacker rolls 3 dice. The defender can opt to re-roll 1 or all his dice, and make the attacker re-roll his dice too?

The Guardian you played Guardian Stance on decides which dice to reroll of all the attack and defense dice, then rerolls those dice. (Minus any that have already been rerolled during the same attack.)