Transfer case drain is a simple drain plug at the lowest point on the case. Fill plug is between case and drum brake. Best removed with a 1/2 inch drive without socket. Make sure you can get the fill plug out before draining lube!
Use a pump devce or plu gun to refill.
Not sure of the spec interval. I do mine annually, probably overkill because of low yearly miles, but it makes me feel better that I see the condition of the lubricant.
The first year I owned the truck I changed the xfer case fluid three times (trying to flush it really well since the fluid kept coming out dirty), but now I change it annually. I put about 15k + on per year for comparison.
I have a question too... I just got back from Moab and my T-case is bleeding on the garage floor. Just a couple of drops fell overnight from the square bottom cover of the t-case. I imagine the sealant has failed and I just need to drain, remove the plate, and re-apply new sealant. Does anyone know what kind of sealant to use? The manual says to use part # STC 3254