Gear whine at speed 55-75

proper oil in transfer case ? most transfer cases have a separate oil reservoir, heavy duty off roading sometimes makes a hole in oil seals and oil leaks out , I allways wash off all my oil seals surfaces after a visit to the MUD PIT saves me a ton on fixing seals

Here are the common causes or rear end noises
Constant high pitched whine at all speeds: Pinion Bearings or Ring gear carrier bearings
Whine at steady speeds or whines on deceleration: Rear end gear set is worn and will need to be replaced.
Rumbling type of noises are caused by wheel bearings or a tire with a separated cord.

It's normal on the AOD transmissions. Their first gear uses straight cut gear and thus has the whining noise. Gear 2, 3 and 4 are helical gears and thus quiet.

Have the tranny serviced with fluid change and filter, if noise it still there it is the torque converter . Have the dealer fix it ,and have it on paper with dealer invoice when it is done. Do all this before warranty expires. IT IS NOT USUAL for any car, Toyota included, to have tranny whine!!!!!

Yes...It can only get worse. If mechanic is a good distance from you, add lube to the diff before going there.

Can you make the noise happen with your foot on the brake, trans in gear, and a little throttle? And does it go away in park & neutral? If yes to both questions you probably have a faulty thrust bearing in the torque convertor. If no to both questions you like have a faulty "input drum to pump cover thrust bearing." Either way the problem must be repaired ASAP or when the bearing fails it will be at the worst possible time, like in the middle of an intersection with a firetruck bearing down on you.

Possible low on fluid will make uniquik whine noise,But another common whine noise is fron serp belt idler pulley,noise will increase with speed.

Just a thought- remove the oil filler cap when the whine is present - (this checks if PCV valve is faulty, causing high crankcase vacuum, which in turn causes a whistling/whine noise as air leaks past engine oil seals). Worth a check.

Try listening with a short piece of hose pipe to locate immediate area/source of noise.

Sounds like your final drive pin in your differential broke causing the main spider pin to tear through the case.Catastropic damage.Check for severe fluid leak under car.If no signs of leaks,check dipstick for greyish blk colored fluid.If this is the case you will need to replace transaxle & flush trans cooler.hope this helps