Drive line vibration

I have 66 Dodge Dart 318 4spd (new clutch ) 8 3/4 non posi rear diff with 355 gears
I have a drive line vibration that come and go
Meaning one time I get in car and drive car to car show say 60 miles away no vibration but maybe same drive home from show vibration may show up or maybe not But !! any thing over 80 mph feel like its about ready to throw a u-join out of it ( horrible vibration)
Joint have less that 1000 mile on them chasing this problem also rear tail shaft bushing and front drive shaft yoke have been replaced
Any ideal ??