1999 HZJ75 toyota troopy ...I have what I think is a gear box noice. Noticeably getting worse over several weeks. Gone from a whine to reverberating whine. Occurs worse when acceleration is off but stll in gear eg. going down a hill in gear or slowing down in gear. Although the noice is starting to become noticable even under acceleration. Nothing noticable out of gear. What do you think? Don

land cruiser
gear box.
steering gear.
manual transmion, what? A/T?
transfer case.
what> gear is bad, name it . what mode of driving, 4wd, 2wd>? 4LO
what?

check that the gear lever is not restricted in the movement and that the pivot pin in the tower for the lever is tight as this will not allow full engagement of the gear. Other reasons for gear disengagement are misalignment of the bell housing to rear of the motor, failing clutch shaft bearing, failing spigot needle roller bearings between the input shaft and the main shaft, tapered lugs on the syncro dog and gears , and worn extension housing bush or if it has a transfer case the a failing bearing between the gear box and transfer case.

Check your air filter housing and the air intake piping. There is a reason why there are high level air intakes available for your vehicle, it is to stop water getting inside the combustion chambers. There is a possibility that your engine has sucked water into the induction system. It needs to be checked and cleaned as soon as possible. Water in the combustion chambers WILL kill your engine.

Take a loom at the manual gear shift. Not the auto one. I mean the 4wd one. I think you will find it is not where it should be. Push it back into place.

These Jeeps have a notoriously bad reputation for having whining differentials in the rear end. You may need to have it rebuilt. But before you do that, please report back with more specific information so I can be of further assistance.

For example, does the noise become noticeably louder when you accelerate, or does it stay the same?

MOST LIKELY HAS A PLUGGED TRANSMISSION FILTER.
TORQUE CONVERTOR CLUTCH FAILURE COMMON.
CHANGE TRANSMISSION FILTER AND ROAD TEST.
IF NOISE IS GONE AND RUNS NORMALLY, CHANGE TORQUE CONVERTOR AND RESEAL TRANSMISSION.

have you checked the oil level in gearbox

No good news on this one I think
Your whining sound problem is the Layshaft bearings in the gearbox are badly worn. It also seems that the baulk rings are damaged on 5th gear.
John

Sounds like the transmission is worn out and will need a complete rebuild. I'd guess $2-3k. Sorry.