From 2004 Wrangler OM (available online at jeep.com):
Schedule "B": 3000 miles/5000 kms for oil; 60,000 miles/96,000 kms for transfer case.
Schedule "A": 6,000 miles/10,000 kms/6 months for oil; 120,000 miles/192,000 kms/120 months for transfer case.
Transfer case needs to be periodically inspected for fullness for both schedules.
The only way to get dirt out of your oil pan is to change the oil. That's one reason some change their oil more often, even when they use "synthetic" oil. The more colder startups, shorter trips, and longer periods of idling for examples, contribute to more dirt in the oil pan.
OP: What fluid does your transfer case call for? I found mine went in and out of 4WD better after a change this year. Mine calls for ATF+4 and that should be good for about 80,000 to 100,000 miles being that it's "synthetic".
My oil change schedule: usually shy of 5,000 miles/8,000 kms using conventional oil and an oversize Mopar/Wix filter. I don't pay attention to the months that much as long as the Jeep is getting some highway trips of 20+ minutes or so.