90 XJ Ltd leaking oil from transfer case in 4wd only

My 90 XJ Ltd 4.0l has a rather odd problem I could do with some help with.

It's got a transmission fluid leak coming from the top of the transfer case at the front, but ONLY when the car is in 4wd drive. Not certain which case it is, but the car has 2wd, 4wd drive high part and full time and 4wd low.

I'm thinking the fluid is coming out of the vent (not certain about this though) but why only when it's 4wd, and more importantly ... what can I do about it?

Thanks in advance for any (helpful) suggestions!

Graham

go to the car wash, spray down the undercarrige drive around for a while than see where it's leaking from could be coming from a number of places.

Check if the transfer case is overfilled that might be the cause of the blowing by the vent tube.

Another possibility is that you only trow oil off when the output shaft to the front is turning (4 WD scenario) and that could signal a broken or leaking shaft seal.

The route is like firehazard said FIRST

SACEM