Vibration in drive line
ON a 78 the drive line might have at least once been taken out for transmission work or even carrier bearing replacement. These drive lines are balanced and the marks must always line up when reassembling and putting them back into the car. But, also check your motor mounts. Strange as it may sound, a soft one of the two (or a broken one) allows the engine to cant just a bit to cause weird pressures on drive alignment upon application of power. And last possibility is the condition of the rear bit of transmission inside the "guibo, ie. donut, ie metalastic". Be sure there is the rubber piece on this stub. Look in a parts book (from Alfa disk) to see the pieces there.