I Bought a 67 Mustang with 3.3LT 3sp MT about 6 months ago and I have had a bad vibration in my drive train. The first Mechanic i took it to said it was my U-Joints and he replaced them but he said that my Rear yoke was slightly worn but did not think needed to replace it. I also replaced all of my tiresdue to they were old and worn out. The Vibration was better but the engine still had a bad vibration and so did the clutch. So I replaced the clutch and had the fly wheel resurfaced and balanced. While working on that I replaced the oil pan gasket and found timing chain had lots of slack. All of these fixed my engine vibration and clutch vibration. But now I still have some vibration at 50 mph and higher. The vibration is not always the same amount it gets worse orbetter when i depress my clutch and seems to be coming from the rear of the transmission.

 

You are going to have to find out what your mechanic means by rear yoke was worn out means. Besides the pinion seal surface, it's a non wearing part.

Feedback with some detailed diagnoistics of your vibration.

Worst at 50? Is it the same in high gear as the next lower gear at the same speed?
When you say clutch in. Is it rpm related or tires/driveshaft speed related.
Tread runout is an often overlooked source of vibration. An out of round rim/tire assembly can be balanced to zero/zero and still vibrate.