Help! New gear noise....

Just a silly thought is the problem in the gears or could it be something else in drive train eg. wheel bearings etc?

Check tailshaft uni's and driveline angles, the uni's are best run at a slight angle rather than straight.
On an older vehicle it can also be the tailshaft sliding splines slightly worn too.

When you engage 4WD a sleeve is pushed over a spline in the extension housing on the front of the t/case. This connects the front drive to the rest of the transmission. In the front extension to the transfer I think there is an adjustable bearing. Was this extension checked, was it over tightened - there is a big nut under where the front drive shaft bolts on. Are these bearings worn. I am only thinking that this part is suspect because the noise only happens in 4WD. Did you really put it in 4WD with the hubs unlocked?
I have owned and recond both an FJ40 and an HJ45 t/case. Good luck.

Thanks to all for your responses so far. Here's a little more info:

1) During the reco of the front diff, the entire front-end was inspected and replaced as necessary. ie wheel bearings, swivel pins etc.
2) When the box and transfer were reco'd all bearings, shafts, shims, gears etc were inspected and replaced if out of tolerance.
3) The noise was not present prior to the reco, so I do not expect the noise to be a result of uni's?????
4) A lot of thought did go into the preload on the tapered bearings in the transfer, which was checked, and re-checked, and re-checked.
5) The mob doing the work is 100% confident that it is nothing forward/rearward of the gearbox/transfer.

I think I can feel the same rumble, although subtle, when in 2WD. However there is some scepticism about this theory, as the amount of load going through the gearbox doesnt change a great deal when 4WD is engaged (the gearbox is still driving the same mass, it's just split front and back). Which leads to the thought that if the noise was from the gearbox, it would be just as prominent in 2WD as in 4WD.

By the way, Im not trying to shoot anyone's ideas down here. Just putting the info out there to help understand the problem!
Thanks to all for your thoughts so far....I look forward to some more!
(Yes I did drive in 4WD without the hubs locked briefly, to test for any alternative symptons...)

A suggestion is to unbolt the front shaft from the t/case and diff , undo the front hubs and drive around. Stop and turn engine off to avoid damage, engage 4WD and drive around again. Listen for your rumble. Repaet with hubs done up. This may or may not help to track it down.