93 LS400 vibration

I am getting a vibration when I go 75MPH and above. I have already had the tires checked and they are a little feathered. The strange thing is that when I do 70MPH and below its fine.
I can even get up to 80MPH then let off the accelerator and the vibration almost all but disappears. Also when on the interstate and I go down a hill its smooths out but when I start back up the hill it comes right back.
Could this be a transmission problem, fuel injector or spark plug wire? This is really bad because I love the car but its really nerve racking to drive on the interstate. It has 111K miles on it.
Stumped.
DS
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Sounds like a universal or CV joint to me. Such a vibration is typically about 4X tire frequency and obviously worst when the driveline is under load. In a car as rigid as the LS you'll likely feel it mostly at specific speeds, when it excites some natural vibration in the car.
-- Pete
David Smith wrote:

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