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The long and short of the problem is that you may only "feel" it at 3500 rpm's when in 4th gear, but that does not mean that whatever is out of balance is only out of balance then. The reason you feel it more at that RPM and gear is because that whatever is rotating and causing the vibration is most likely hitting it's Resonant Frequency. Resonant Frequency is when the natural Frequency of the vibration aligns with the natural frequency of whatever was dampening the vibration above and below those frequencies. Think about pushing a child on a swing. Push the child at the perfect time, the child will go higher and higher, However, if you push the child too soon, or too Late, maximum height is not achieved. My guess is the vibration is directly related to the speed of the car and really nothing to do with what gear or RPM. Sometimes it doesn't mean anything is really wrong. Sometimes things just line up to cause Resonant Vibrations. I would determine the speed that the car is going at 3500 rpm in 4th gear. Say 65 mph for an example. I would go 65 mph and coast, does the vibration stop, dampen, or go away? Like I was saying, it doesn't mean something is wrong no matter how annoying it may be. As an example, if you had a tire that was not balanced and vibrated bad at 60 mph, just by changing the shocks, it might go away. By changing the shock, you are changing the Resonant Frequency of the tire rotation/suspension assembly. Now the tire would still be out of balance but by changing the shocks, you may eliminate the tire from hitting a resonant frequency.
 

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I chased a driveline issue for years as well. I bought a magnetic digital angle finder and thought I had driveline angles perfect. Even wore out a set of ubolts changing out shims. Decided to do a leg of the power tour and loaded the trunk with some stuff, way more than I ever hauled normally and the rear just sagged too much so I threw on a set of air shocks. The point of this whole story was once I got home and emptied my trunk, I forgot to let the air down on the shocks and to my surprise, just that tiny change in pinion angle eliminated my vibration. I put the QA1's back on and tried to find that sweet spot by changing the shims yet again, but never quite got it perfect.
I changed from Muncie to tremec and I got this same type of higher speed vibration that only happened above 60 mph. I downloaded an app from tremec for iPhone that helps you find correct angles actually using your phone for input. Worked for me - It ended up needing a 2* more downward angle. Prob solved.
 

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I once had a similar issue with my 73 Ventura. It didn’t start until i replaced my turbo 350 with a 400. I tried a bunch of different things to no avail. It turned out that my solid transmission mount was the culprit. I replaced it with a poly trans mount and the vibration went away. This may or may not be your issue but it’s a cheap/easy thing to try at least.
 

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Want to mention this often overlooked simple fix for some chasing vibrations, if you have aftermarket wheels chances are they have an oversized center so they are considered lug centric.You might try Hub Centric Rings to true them up. I have a friend that had a vibration that he chased for years untill a young guy at the tire shop suggested the rings and that fixed it…. Just cost about $20.00 or so.. hope this helps..
 

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Another possibility is the trueness of the tires? A tire that is not true can be compensated for with weights when it’s on a spin balancer. But once it’s put back on the car, the physicality of the true center of the tire may differ from the true center of the wheel?
 

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Want to mention this often overlooked simple fix for some chasing vibrations, if you have aftermarket wheels chances are they have an oversized center so they are considered lug centric.You might try Hub Centric Rings to true them up. I have a friend that had a vibration that he chased for years untill a young guy at the tire shop suggested the rings and that fixed it…. Just cost about $20.00 or so.. hope this helps..
I had never encountered this before but it makes perfectly good sense. I watched this video to learn about it.

 

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My 66 Nova SS has had a high speed vibration since I bought it in Wisconsin and drove it home to Calgary about 15 years ago. I have since,rebuilt and balanced L79 engine,replaces clutch and pressure plate,replaced muncie with a superT10,replaced driveshaft and joints,replaved rear suspension springs with multi leaf,changed wheels and tires to 15" American Racing Mags. I still have the vibration in fourth gear at 3500 RPM. Does not vibrate in 3rd gear at 3500 RPM. I'm thinking I should replace my rear axles and bearing. What do you think?
BTW, my drive line angles are correct
Put it on some face stands and run it up to speed of vibration while someone has a look see
 
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