> again then said that the noise appeared to be coming from the
> differential. I explained that I was headed to an off road trip in
A common problem with the factory pinion bearing on the rear diff is that it loses the hard facing and develops a whine like you have described. About 1 in 4 cruisers have bad bearings and Toyota will not replace them under warranty unless you jump on them. I've had them replaced on every 80 Series I've owned and whilst total bearing collapse doesn't occur until over 120000 km, the whine only gets worse.
ARB hold stocks of these bearings here in Melbourne because they are often pitted from new. They replace them when the lockers are going in.
A tell tail sign is notchiness in the rear pinion or vertical movement of the pinion right on the driveshaft flange.
Check it out by jacking one rear wheel off the groundand secure the vehicle (handbrake off), get under the vehicle and rotate the rear driveshaft. If it's not smoooooth, then there's your problem. Ditto for any vertical movement.
Same goes for the front bearings.