cause of the 3k rpm speed vibration, a 15 dollar u joint. But what a complete drive train exam and repair adventure he went through. That car is going to be set for a LONG time to come

the scoring is obvious, but TWO driveshaft balancing companies didn't catch it, as everyone merely did the wiggle test for slop. It didn't have any. 

Yes, this was what EVERYONE told him to check first as the vibration only occured at a certain RPM, when in gear, not in neutral/clutch in. But, while spending weeks trying to find this, a LOT of other parts that needed to be replaced got attention... from the engine, trans, brakes, bearings, steering, etc. 

This Mustang is not extremely well, sorted out, and my cousin now know a LOT about his car that he'll instantly know what to do with if it ever needs attention. 

Mustang Vibration Diagnostic... 

5th gear accelerating getting up to roughly 70mph, car starts to vibrate through the floor and the shifter.

 Push the clutch, it goes away, release the clutch it comes back

 Continue accelerating 80mph, push in the clutch, vibration stays, release the clutch vibration stays 

 Let off the gas, decelerate, 65mph, vibration stops, cruise this speed smooth as glass.

1967 mustang, 289, T5, 3.80 diff 

Parts replaced

 Harmonic balancer, engine mounts, transmission mount, wheel bearings, brake drums turned, axle shafts straight, throwout bearing, pilot bearing, clutch/PP, tunnel trimmed, driveshaft lengthened and balanced at 3k rpm, correct 28oz flywheel