I would be checking rear diff.
Have a look for leaks, check level.
Next thing ensure pinion nut is tight.
Some Toyota's are known for them loosening.

 

great to hear that would you please tell me how’s your car performs after this flush, since I’m going to use Amsoil signature series and I found a qualified Lexus mechanic will perform a machine flush and ask me to bring around 16qrts, I have 2008 Lexus GS 350 with 111mi on it , I was reading here and there about flush not a good idea with high mileage but I heard also good results as well, most of them they recommend me doing just drain n fill which will drain only 3qrts at a time !! Would you please give me your honest opinion should I go ahead do the flush or should I stick with the drain n fill ? By the way the mechanic took the car for test drive and told me your transmission shift smooth and I don’t see no problem at all, but I told him the history of the car says it’s never been had any transmission service! Thanks for your time looking forward to hear ya

 

I know it’s been along time since you post this but I’m wondering how your car perform after switch to Amsoil signature series ? And did do you a complete flush or just a drain n fill ? Please share your experience since I’m really confused and not sure should I do the flush or just drain n fill, my car has 110mi now and it’s never done any trans service before ! Thanks for your time

 

I wrote a DIY #2IS thread on this topic if followed to the letter, one can dump more than 1/2 out in one flush. See below.
PS Amsoil went back in mine also.

2is-transmission-fluid-change-diy-a760e-rwd