George asked: has it been suggested to check the bushing inside of the front drive shaft that the "olive" on the output shaft rides inside of?
Thank you George - yes it has been changed - it probably has 50 miles on it (max) and is now coupled to a reconditioned transmission with a new olive - great suggestion, but I don't think that's it.
To your other thought, yes the splines are well greased and all seem to move well - and when I tried the other rear-section (i.e. different splines), the vibration was so similar it made me think the splines couldn't be the issue.
PSK asked: The 2 driveshaft pieces have to be aligned don't they?. Are we doing this?
Thanks, yes we're doing that:-) The driveshaft has been balanced twice now..once by Frank Wallace in San Francisco - Frank hosts driveshaft balancing tech-talks at his shop and most certainly knows his stuff. Then I had it balanced at the famous driveshaft shop in San Jose (name escapes me..) and he re-tubed the rear section and re-balanced - it came back to me carefully marked for how to re-assemble the halves and I always do so. When I tried the alternative rear half from another shaft for comparison, I couldn't do that of course - but the vibration was so identical I decided it coudn't be a problem with the rear section. I do now want to try with a completely different front section - but I don't have one. I only have one with the 1969 and later beefed-up center support and I don't know if you can switch them...hence the original name for the thread.
PSK asked: What about the centre driveshaft support. Have we replaced that?
We have (twice). every replaceable part.
SprintGTC said: "it's bent" and he fixed it. The vibration / weird harmonic disappeared completely -for good.
Right - that was always at the back of my mind - hence my visit to the 2nd balancing shop - I was very excited when he re-tubed the rear, thinking he'd discovered a 'bend'...but it made no difference - I have to try a new front-section - thanks for the recommendation for a shop. Funny, I'll be in Spokane next month....
This is really a thread jack
Yeah, but it sounds like a great subject for a brand new thread.
Thanks all for the replies - all idea-bouncing is extremely useful - I will try more stuff this weekend and report.