Transmission/xfer case/U-joint noises?

Hello,
I have a 1998 Jeep Cherokee 4WD - 6 cyl. 4.0 liter engine, auto transmission. I have recently been experiencing, or rather, recently noticed a strange sound that appears to be coming from my transmission or transfer case. The sound appears to only be heard when the following occurs:
1. In 4-wheel drive
2. In drive (if I take it out of drive with 4WD drive engaged, the noise stops).
3. Moving slowly (under 20 mph). Once I get up to speed I can no longer hear it.
4. It seems to be louder when driving up hill.
5. It sounds, to my best decription to be a chortle... almost like a slow turkey call, if that makes any sense.
6. It also sounds if it may be related to the suspension, But why only in 4WD and why only in Drive? (A buddy helped me replace all of my U-joints about 3 years ago).
7. I experience absolutely no performance loss. It runs as great as it ever has.
Thanks for any information regarding this inquiry.

Could it be a slight rubbing of the t-case chain against the case?

Put it up on 4 stands--a lift would be better--and run it in 4wd and use a stethoscope or vinyl tubing to listen at the t-case.

Thanks Joe,
I don't have a lift, but a friend of mine has stands. I will talk to him and see if he can help me out. If it is the x-fer chain rubbing against the case, is this something I need to take care of? Thanks again.

Jon

Yes--they can and do wear through the case or they will start "jumping" under load.