Front drive train noise

I went up the beach on the weekend and heard some unwelcome noise. All sounds well in 2wd but when I shifted to either full time or part time 4wd I got what I thought sounded like front end "vibration noise". I couldn't really feel anything different, it just sounded like vibration. (I think that makes sense.) When I dropped it back to 2wd and stopped to pump up the tyres again the noise dissappeared.
Now I can understand why I might get that noise if the uni's were a bit dry or tired, but what I don't understand is why I only get it when I shift to 4wd. I thought that since these things are spinning all the time anyway, they will fail in 2wd as much as they will in 4wd.
Does this sound typical for uni's or front drive shaft, or should I be thinking of something else all together. I'll probably take it down to Hammonds for a service/check soon but any ideas would be welcome.
Thanks.

[Snapey]

I think you drive an XJ Wheelie? Had similar problem a while ago - turned out to be no more than a stuffed passenger side engine mount. Seems the extra torque when in 4WD, combined with the 1" transfer case drop, was enough to pull the engine over to the passenger side (movement thanks to engine mount) where the fan was able to touch on one of the metal air con lines. Made a hell of a racket. Easy to check - just take a torch and have a close look at the engine mount. It may seem OK as mine did, but closer inspection showed a large crack in the rubber under the through-bolt. Check the edge of the cooling fan itself for signs of rubbing on something. You may even be able to fix the problem with a well placed cable tie.

[Nevermind]

Could just be the noise of the transfer case doing it's thing, then again could be something else. Might be a good idea to get under it and have a good look at the uni's both front axle and front shaft, give em' a good wiggle see if you get any movement. No doubt the boys at hammonds will figure it out if you can't.

[tonyl]

I agree with nevermind

I thought they would be the most likely culprits but snapey's post is interesting. My engine mounts are new but the idea is still worth checking. (Yes snapey I do drive a 97 XJ.)
What I still don't understand is why I only get the noise when in 4wd. The uni's always have torque applied to them; either by the wheels or the transfer case. I don't get why only one of these situations causes the noise.
Any ideas? Is this typical?

[tonyl]

chain driven transfer when you engage the front wheels it is quite noisey sounds like a vibrating noise if you have no vibation I doubt you have a problem but never hurts to get checked out

[Snapey]

I agree with the other guys re the normal transfer case noise - but mine makes a distinct (although not overly loud) wurring noise - and only in low range. The mounts thing is just a simple place to start. Since Wheelie says his mounts are new this is much less likely. I am getting to the point where if I try to chase every little rattle and bump in the Jeep I would go nuts - so unless it's obvious I'll find it when something breaks!