i've removed, disassembled and resealed many transfer cases. no need to replace any parts just because of a leak where the case comes together. if parts are worn its obviously the best time to replace them.
you do have to remove the case, and you do have to disassemble, clean surfaces, reseal and reassemble the entire case. and it is just sealed with rtv where the cases come together, there will be a couple output seals you'll need as well, and iirc a seal on the back of the trans and the front of the t case you may as well replace at the same time.