380 SLC vibration

[ERM]

I am looking for help trying to diagnose a vibration.I am working on a 1981 380 SLC (European model) with a 4 sp.auto model 722 304.I get a vibration at about 45-50 mph,the vibration gets worse a the speed increases.The vibration ONLY happens in 4th gear, if I downshift 1 gear it goes away no matter what speed, as soon as it shifts into 4th gear the vibration comes back.This vibration does not seem to be tied to the engine speed.There is no vibration in neutral,or in park.Here is a list of what I have replaced.
motor/trans mounts 2 years ago
both flex disks/rebuilt & balanced driveshaft 1 year ago
rebuilt axles 6 months ago
The transmission has no other issues ,shifts smooth,no delays going into reverse or drive,no slipping or flaring on up-shifts.Could this be a torque converter issue ?Any ideas?thanks