violent shaking past 45mph

[FireFight]

Driving my 24v car to work today, and when I hit 45mph it started shaking, not driveshaft thudding like a bad guibo or CSB, but shaking like the wheel was only on by one lug bolt that was on finger tight. and the faster I went the worst it got.
so I limped it to work, but found that it wasn't a RPM thing, I could take it up to 7000rpm and nothing, it was a speed thing.

when I was almost at work I decided to get it up to speed again, but it wasn't there.

Jacked up the front of the car and found both front wheel bearings are nice and tight.

My tires do look low, so maybe when they where cold they where even lower, but I don't imagine that causing that about of shaking

Both axles looked fine.
any ideas?

[JIM1inWisconsin]

Sounds like it's a bad bearing along the drive shaft. I'd get that checked.

[dnguyen1963]

There is a thing called harmonic imbalanced. Many tires, when not properly balanced, can exhibit shaking at certain speed, i.e. certain harmonic.

[7pilot]

If you can feel the vibration thru the seat, and the whole car is vibrating, move the rear tires to the front.
If the vibration is now a steering shimmy, then one or both rears have broken belts.
m

[e30rapidic]

are you using spacers? hub centric?