Realistically you can use what you like, I think the question is, what will happen if you use 75W90?
Certainly 75W-90 is one of the more popular gear oil weights for Toyota/Lexus, however, like 5W-30 was the oil of choice for a long time, now here we are at 0W-20.

Chances are you will probably be ok, but I have to assume there is a good reason why Lexus specified what they did (it could be they just wanted to improve MPG...gonna maintain that CAFE rating).
I can't imagine the diff. will explode because you put 75W-90 in it, but when I look at the cost of the oil versus a diff. rebuild, hands down getting the right oil is the way to go.

Having said all that, if you do go with 75W-90, i would use something fully synthetic...that way it will be as close to a regular 75W-85, without actually being it.