My 67 has a pretty substantial vibration at highway speeds. U joints are good but I think it’s possible I have a driveshaft issue. I think the driveshaft may be too short. What should a driveshaft measure from center to center of the u joints?
You did not specify engine and transmission, but for a 1967 with a 289 it is 54 3/16 inches,
with a 390 and automatic 49 41/64 inches and for a 390 and manual 53 and 1/32 inches.
You might have to turn the drive shaft 180 degrees. My Eliminator was vibrating very bad at 70 mph it fixed the problem. This is why you should mark the drive shaft when taken off.
I would doubt if it is too short.
Look to see if a weight has come off. It would look like a rectangle bare spot with 2 spot weld marks.
If you disconnect the rear u joint , the drive shaft should move forward 1/2 to 3/4 inch before bottoming out in the trans . On our 67 , I ended up making some custom angle wedges to improve the drive shaft angles . Mercury made some different angle wedges but I could not find the one I needed. Note…mustang did not use the cast angle saddle and rubber isolates. My shake tured ot to be crappy aftermarket drilled and grooved front rotors…40 grams out of balance
My car is a 289/c4. My driveshaft measures 53 from the center of the joint. And it appears to have 2.5” before bottoming out.
Replaced with a 390 stick driveshaft ??
You might consider going to a good driveline shop and hav them build a driveline tht fits. Also id guess the output yoke bushing is toast. A good trans house can change bushing with the tailhousing in the car
I replaced my bushing and seal in hopes of making it better.