We experience a lot of road noise in our 2007 RX 350. Very loud, rear passengers complain particularly. We put on new top of the line Toyota tires 235/55R 18's. Still emits a high pitched roaring from the rear end. Could it still be the tires?

I am thinking that the noise is from a failed wheel bearing or possible cv joint failure
find yourself another service dealer as tire noise is either there or not there and rotation (if it is a faulty tyie ) would move to the new position
Here is a clue to consider
rotating tires is a ploy by the tire dealers to sell tires
have a wheel alignment done on the rear first and then the front and leave the tires alone
if it is a front wheel drive vehicle you will get 3 times the mileage out of the rear tires compared to the front drive tires and that is a big saving for you
next point -- radial tires continue to wear in the same pattern regardless of where they are put so how are you to know it it is a faulty tire construction or a need for a wheel alignment
third point --- many brands of tires are made as directional tires ( a directional arrow on the side wall) and cannot be rotated because they cannot be fitted on a wheel that rotates against the arrow
if they are the wear in pattern of the belts in the rubber is altered and may lead to tire failure ---the tread pattern is designed to remove maximum water and produce maximum grip in the direction of the arrow

Replace the shock absorbers front and rear.

rear wheel bearing.

Sounds like it might be a damaged rear end. Chock the wheels or put rear differential on jack stands and put vehicle in neutral then by hand turn the drive line back and forth and see if there is any loud noise when u do. There should not be a lot of play. if on jack stand you can also turn one of the rear wheels and see if there is any load noises or excessive play

It sounds like a possible wheel bearing failure.Get it checked at a local repair shop ,or possible loose bearing hub.

ry checking the condition of your wheel bearings they may need servicing or replacing and cv joints

A few broken ring gear teeth will cause that. BUT it wouldn't go very far till it made a whole lot more noise. The transmission will make lots of noises and can cause the same symptons you are complaining about. If it's 4x4 try and put it in four wheel drive and see what happens, don't drive far, that will help narrow it down between trans and rear end. If it's not 4x4 then if you can, remove the rear drive line, jack up the rear tires so they are off the ground and try and spin the rear pinion drive flange (the thing that the drive line was bolted to). It should spin fairly freely (it will have some drag) if you feel any binding or grinding then the problem is in the rear end.
Report back with your findings so we can better assist you.
P.S. both will be a big job and unless you have experiance working on these type of jobs then it will most likly be best left to someone who know what they are doing.

caster,camber and, wheel bearings
have your car 4 wheel aligned this will adjust your rear caster and, camber.
have your wheel bearings checked

unless you are off road or it snows alot where you live I would recommend MICHILIN or CONTINENTAL touring style tire

put the wheel off the ground and turn it.there should also not be any play holding the wheel on the top and bottom moving it.put the back tires on the front to rule out not having a broken belt in the tire,which can sound like a bad bearing