HKS,

Yeah, it's weird but it happens. Just happened to my 99 GS 400 with 19" Lowenharts and brand new 285/35/19 Bridgestone Turanza S03s. I couldn't figure out why it was "flat" every time after I drove it. Then I took it to a tire shop and they discovered that the beads weren't sitting on the lip of the rim. They added bead sealer and remounted the tires. So far so good. Luckily, it wasn't bent rims or a big hole in the tire.

 

If you have a Discount tire in your area go there and have them do What they call a "road force balance"
I know your probably sick of getting balances cause I was in your same shoes.
I too had them done and the vibe was gone for a bit only to return.The road force balance has taken care of it and it has not returned for about 3 months now.
The road force balance will also tell you if the wheel is bent.Its a pretty handy little tool.Give it a shot.

 

Im too lazy to read ALL the posts but I read a few. I used to be a tire jocky so I consider myself an authorty on this subject. First and formost if the rim/tires were road force balence then the rims are NOT bent. The machine actually measures the rims all the way around for this type of balence and would warn the operater if the rims were overly bent. Water, flat tire repair crap, or somthing else INSIDE your tire will cause a problem that is hard to track down. A balencer will sometimes detect this but other times it wont. Here is what you need to do. Take it to a shop where most of there business is tires (kaufman tire, where I worked, is a good example) and have somone there take a look. Make sure they take the tire OFF the rim and look inside. Next falkens do NOT ride good. They have a stiff sidewall and after every install we test drove and falkens just ride bad. Next, if its a tie rod (all cars have these) then it can be detected by putting a car on a lift and trying to wiggle a front wheel left and right (like you were trying to steer). If it move too much or doesnt match well with the other side then that is your culprit. Good luck.

 

I had 17" OEM chromes and had constant problem with vibration from 55-65Mph, sometimes til 75Mph and very loose/jerky steering.

About a month ago, I replaced lower ball joint which turned out to be bad. This reduced vibration some what and cured loose/jerky steering. I then replaced my wheels and no vibration what so ever. Rides beautifully and I am very happy with it! I had forgotten how my car used to ride.

My conclusing to this issue is: replace lower ball joint (I had 68K Mi) and to replace the wheels. I think 17" OEM wheels are hopeless based on my experience and others on this forum.
Think back how many times you had to take it in for balancing, maybe good for 1-2 days and the vibration is back (i had taken it in for balancing 10+ times). I don't care if this is Lexus or Bently or any other brand. This is not normal.

 

With 50K miles, a few dollars on a good suspension shop might be in order. Since you have four wheel independent suspension, all the moving parts need to be inspected for wear. This includes ball joints, tie rod ends, strut and strut mounts, link mounts, spring mounts, axle shafts, sway bars, wheel bearings ...everything. A good shop can properly and correctly check these components. Any one or combination worn can cause a tire to wear improperly under normal slow driving that you don't feel till 55-85 MPH, when loads and stresses are compounded.