The cover in the front diff is extremely difficult to remove. So your best bet would be to purchase a suction punp from your local auto parts store for about $10. Remove the inspection plug on the back of your diff, insert the suction tube and draw out as much of the fluid as you can. Move the tube around to try to get as far to the bottom as possible.
Then refill with new fluid until it just drips from the inspection hole. Reinstall the plug and you're done.