If the transfer box is leaking you should be able to see it easily if you have a quick crawl underneath. The usual suspects are the front output shaft seal, rear output shaft seal or the seal between main box and transfer box. Well worth checking the breather isn't blocked before paying for new seals - blow it through with an air line to clear it - pressurisation of the transfer box when warm can cause your seals to leak, but this won't happen with a clear breather.

Cheers,