I also had vibrations problems with my 91 LS from 70-80 MPH. It has a little over 200k on it, yes. I just want to give everyone a good heads up about dealing with the vibration. I bought it about two 1/2 years ago with 80K miles. It began having trouble about a year after I got it.
First, I was sure it was the suspension. BUT, I have replaced every suspension piece on the car with FACTORY Lexus parts. I went from every front end bushing, bearings, springs, struts, upper control arms, lower control arms, and every piece that attaches to the suspension. Total parts about $8000
Next, I thought maybe the transmission or engine mounts. Replaced all of those with FACTORY Lexus mounts. Total parts about $600
Then, I thought well driveshaft, differential, or maybe axles. Replaced all of those with Factory Lexus parts. Total parts about $3400
Then I thought about the wheels/tires. Got a set of Helo 17" Satin Chrome wheels, with Goodyear Eagle F1 Tires. Total price $2500
Finally, after more of these antics, I wound up with rebuilt engine that was bored over, head glassbeaded to flow 80% more air with a three angle valve job, Rod Millen Intake, Rebuilt tranny with Rod Millen Valve Body, computer reprogramming, New AC compressor, evaporator, condensor, expansion valve, drier, PS pump, rack and pinion, alternator, and then full dualed out exhaust with race cats, 3" pipes, 2 Forza Flow mufflers, middle cat removed, and resonator 3A Racing tips (pencil style that resemble the originals but are slightly larger) Oh yeah, and nitrous. Total price $Don't Ask.
When I got done with it all, the vibration was gone. Just a craptastic spark plug and a faulty distributor cap that only missed when it was at a certain RPM and was moving and a slightly cracked flywheel.
FYI check the cheap stuff first, but man do I love my car now. I also put a really nice stereo system in it
Pioneer P720 Premier H/U
Pioneer 150x2 Watt RMS amplifiers (2)
MB Quart Components w/ crossovers, tweeters, and midrange (4) (8 speakers total)
Rockford 800.2 Punch Power Amplifier
MX Autotek 1222 subs (2)
1 farad capacitor
XM Radio
Navigation Unit
I would recommend buying a car with vibration troubles, it worked out for me. I never would have spent the money to make my car so damn enjoyable and now I don't think I will ever get rid of it unless it gets stolen or totaled. (Seriously)
BTW, none of my problems can be attributed to Lexus. The car functioned as a drag strip racer, road racer, tow truck (I towed 2 different Ford trucks, twice each about 50 miles), mudding vehicle, a car that would fly (and I mean that in the aviation sense) over the crests of any hills or large bumps in the road (Dukes of Hazzard style, no joke), and most importantly, my daily driver to work.
Now I am being dead serious about how poorly I treated my car (I have video). But nothing honestly ever broke. It never once didn't start or wouldn't drive. I replaced everything only preventatively and had no other reason for fixing everything.
The only thing that broke from all of my abuse was the flywheel. My garage is filled with the old parts that are still good. So I refuse to believe anyone who says that their Lexus has something seriously wrong with it. If there is any problem with a Lexus, it is the person or place that services it. I did almost all my own work.
Also, just so no one leaves this post crying about some poor LS400 getting ragged out, I no longer tear it up and it is in near mint condition. So I know first hand that you can repair ragged on vehicles. I honestly do not think the way people treat these cars matters much at all. They just seem to go forever.