Beginning in 2006, the Front Differentials are factory filled with Synthetic, so you should already have the white cap.
Jperry, Grape Juice is the nickname for the GM Synthetic Axle Oil. The part number is 12378261 and retails for $27.55 per quart from GM. You can use this oil if you like, or any good synthetic 75W-90 or 75W-140 if towing heavy.