need help with transfer case question

[scot sund]

i have a 2000 expedition and my front output shaft seal is leaking on transfer case and in a repair manual it says except the new venture transfer cases ford procedures state that the front output shaft seal is replaceable only on disassembly/overhaul of transfer case assembly is there a reason why or can i just replace the seal itself, i dont want to rebuild it just want to stop leak
does anyone know why this is its on the autozone repair instruction page