That is not correct info. The 2017 owners manual says 75W-90 in the differentials, and 75W in the transfer case. Your service advisor may have his head where the sun doesn't shine. 75W-90 is not used in differentials without a limited slip additive...no matter as the series 200 trucks don't have limited slip diffs. Toyota recommends different fluids between diffs and transfer case.

That's the same as my '13 Land Cruiser.

 

The 2017 manual from Lexus states:
Toyota Genuine Differential Gear Oil LT 75W-85 GL-5 or equivalent

Toyota Genuine Transfer Gear oil LF or equivalent SAE 75W

There's no viscosity specified for the transfer case so I had to put my faith in the service advisor. It does have a Torsen limited slip differential.

 

Trans case viscosity is 75W. Torsen diffs use worm gears v. the clutch plates used in most other limited slip diffs...without clutch plates, no limited slip additives are needed.