Been away for a while as I just moved from Tucson to San Angelo TX.

I had the Firebird shipped, which turned out to be a good idea as I discovered something while testing her out on the highway to see if she could endure the drive.

I have never had her over 40ish MPH until now, so I have no idea how long this may have been going on. I have an "idea" as to what it may be, but I would like some opinions from more experienced car guys/gals if you have the time.

My car is a '69 Firebird convertible, modded out a lot to include transmission (TREMEC TK-500) and ('71 455 450hp aprox.)engine. I doubt anything in the drive-line is stock, I bought the car with the engine and trans. I still have my cocktail shakers.

Here is the vibration description:

No vibration under 40mph
Vibration starts around 40-45 and is "scary" at 65mph.
Vibration is less severe if I have more "tension" on the drive-line... (going down hill it is worse, braking & accelerating it lets up a little).
My alignment "seems" good, if I take my hands off the wheel it is steady and does not pull to either side. There is also no odd wear on the tires.


Thoughts? I lean towards the drive shaft balance... suggestions?

As always, thank you!