Unless the recommendation has changed from '01 to '06, Toyota calls for 7wW-90 in the xfer case and front diff, and 80W-90 in the rear diff. My choice is any good brand of synthetic 75W-90 in all.
Take a good look at the fluid, especially in the xfer case and front. If they look like new, they probably are as good as new, and there's not need to change unless you want to go syn.
While you're at it, change the power steering fluid and brake fluid along with everything else. They all deteriorate with time, heat, and moisture.