The diffs can go 30k under normal conditions, 15k for heavy duty use. The rear is easy to suction out, although most of us take the diff pan off and clean it out/reseal. The front dif and transfer case need a 14mm hex wrench, but otherwise is pretty easy to drain out and refill.
I would wait until 30k for all of this. DIY, it's way too easy a job to let the dealership screw it up and overcharge you. I assume you have the 4.7. Use synthetic gear oil for front and back, no additive needed. get the transfer case fluid at the dealer, take your vin in and make sure it's the right stuff.