Many thanks to all for your input(s).
Haven't been able to do much because the weather has been unusually rainy & cold. Spring hasn't quite gotten to Germany yet.
Been thinking of removing the two piece driveshaft and center bearing and bringing them to a specialist. They tell me that they can check the straigtness, U-joints, bearings, etc. and get the shafts back to me within a week. Sounds good, but they are a long way off and it is quite expensive.
I have given up the idea of going to a one piece shaft.
Today,I was thinking over my options and have come to the conclusion that the vibration problem first became noticeable after we rebuilt the transmision and replaced the clutch. When I checked the phasing of the U-joints, I thought I had found the problem. After correcting the phasing, the vibration diminished. BUT, it's still there! Albeit much improved.
I am now wondering if maybe I have a driveshaft balance problem. When I corrected the phasing - I may have done it incorrectly.
If I remove the rear shaft and rotate it 180°, it will still be in correct phase but maybe the balance will be better. Does this make sense? Has anybody been there??
