Intermittent oil leak 2002 Jackaroo

We have isolated the source of an oil leak that occurs during highway rather than city driving. It's in the high /low range selector. Because the oil leaks onto the front drive shaft there is oil everywhere from that point The actual oil level in the transfer case has not dropped markedly but it looks bad
We are also having difficulty finding a service manual.

Is there anyone out there with info or suggestions?.