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I would go ahead and fill it now, then set it out of the way until you need it. Mark it to remind you you have filled it.
If it's a new diff, install GM's differential brake additive along with the lube. Easiest way to get this stuff is to order through a GM dealer in your area.
Fill it, rotate it in your hands to we all internal areas, turn the pinion shaft four time to guarantee lubing the diff gears and internal diff housing parts.