The 03' Limited has 88k miles and my 06' limited has 64k miles and i have never done the t-case or diffs on either Jeep yet. The 03' is going in possibly this weekend. Ill have to see what place around here is the cheapest to get it done at.
i usually let the front go a little longer 18k-20k unless i am bored and have time to do both at once. but the rear has a rubber cap and water does get in there. everytime i drain it the oil is contaminated.