I have a 79 Corvette with a vibration at 70-80mph. It seems to oscillate between barely noticeable to slow down the car is coming apart. It has been in the car since I’ve owned it (about 8 years).
I have replaced all tie rods, a-arm bushings, ball joints, all 6 U-joints, tires, front and back wheel bearings, pitman arm, idler arm, sway bar bushings, brakes and calipers, rear suspension bushings, tires, the rag joint on the steering column, disc rotors, numerous alignments and balanced the drive shaft.
I am at the end of my rope. Everything I replaced needed to be replaced, so the money didn’t go to waste, but the problem is still there.
Anything I missed? I need help!
harmonic balancer on the engine maybe?
CV Joints?
Sorry, I replaced it 6 years ago with the engine and transmission mounts.
The car doesn’t have CV joints. It uses a drive shaft and 2 half shafts. All 6 U-joints replaced with Brute Force U-joints. I’ve also checked the runout on the rims and they are all within 0.005" of true.
What about the center link; sometimes called a drag link?
Just a question. How is this problem if you shift into neutral at 80 MPH and allow it to coast down? Or have you tried that?
Just trying to determine if this is related to engine performance instead of the chassis; and I did run across a Ford one time that exhibited some serious shaking going on but ONLY if a steady speed was maintained with a feather touch and also on deceleration. The problem with the Ford was a corroded coil wire end believe it or not. At idle and on acceleration it ran like a brand new sewing machine.
When I did the front end I replaced the drag link too. The problem is still there when coasting in neutral. It is even there in hard cornering.
Also, I replaced the engine and trans mounts. This has me baffled. It seems to be coming from the whole car and is also noticeable through the steering wheel. I even had the frame checked for straightness. It has had me stumped for as long as I’ve had the car. Sometimes when I drive it it is barely noticeable and other (most) times it feels like it is coming apart.
Back in Vibrations class in engineering school, they taught us that some cars, particularly convertibles, require anti-vibration oscillators which are little spring-mounted weights attached to the body in strategic locations. These oscillators are hidden in the body work, but if you could see them, you would see them vibrating like crazy at certain vehicle speeds to cancel the natural vibrations in the chassis. On a 3-series e36 BMW convertible, for example, they use the battery as the weight and mount it on a spring. They have to use a special battery to tolerate the vibration, because it shakes like crazy at certain speeds.
If I had your problem, I would do some homework and see where the oscillator(s), if any, are located in your car and I would find them to make sure one is not broken or missing.
P.S. Trivia - According to my Vibrations instructor, the first car where this technique was applied was the '57 T-bird convertible. It was considered to be quite the engineering coup, because it was an easy mid-production fix that could be retrofit at the dealer and saved them from a very costly recall and redesign.
I ran into a similar problem once and you’re not going to believe what the problem was; the torque converter. I was on the same track as you by swapping tires, rims, balancing driveshaft etc and had hit a dead end. The vibration was somewhat random and varied from subtle to scary like something is coming apart. It was always worse on power application so I finally decided it might be the TC because it was a high horsepower engine and semi-automatic trans that took a pounding occasionally 
Swapped it out and immediate relief. If you have an auto trans, it might be worth doing based on what you’ve done so far.
BTW- the trans fluid looked fine w/no abnormal debris in pan.
I’ve never heard if Vibration dampers on a vette but I’ll do some research and look. The inside of the body and most of the frame is fairly accessible so I’ll also go over it from bumper to bumper. Thanks, I’ll post with the results.
I thought the same thing and instead of just replacing the TC, upgraded. I removed the TH350 and went with a 700R4 conversion from Bow Tie Overdrives. Excellent choice, it jumped my mileage to 22mpg highway and it launches like a rocket, but still has the vibration.