Had one of the stock engine mounts separate (after 40 some years!!) so replaced it with caged urethane high performance type mounts. VIbration is substantially less now, so I'm convinced it is engine related. I've ordered a fluid damper to replace the harmonic balancer, and I'm optimistic that this will do the trick.

Going to finally replace the rope seal as well, since it's leaked since day one and is now POURING oil out the back end! THis time I'll use a rubber seal!

 

For the record, I stumbled over a rim the other night with marks of where a wheel weight use to be. Since I bought them new they shouldnt have had any wheel weights before then.... and I now remember the time it flew off... that moment when you hear a bang and never figure out what it was. Thats what it was. Replaced it and it helped even more. Gonna try plugging the axle access holes this weekend to see if the last 2% of vibration dissapears. So far my vibration was a combination of motor mounts, trans mount, drive shaft, and lost wheel weight. Any one of these would cause the stick shift (its a 5-speed) to shake at around 60mph

 

My car used to shake like no other, but it was the ball joints[&:]

I'm no genius, not even good with cars yet, but any loose motor mounts or something that could let the engine wobble up front? (just read the post above mine - part of his was motor mounts!)

I know you said you checked it out of gear, but just idling, if something's loose will it shake?

If that was a stupid suggestion, sorry

EDIT - for the second time
Nevermind, seems like you narrowed it down...I didn't read all 3 pages before posting [&:]

 

Did you ever resolve your high speed vibration issue. I have a similar problem that I am just now beginning to delve into on a 1968 Mustang coupe with a 289 V8 and 4 speed manual transmission.

Thanks

[IMG]local://upfiles/93974/0CF15DC16D214402B40E0DE1EEC8300A.jpg[/IMG]

 

better off to start a new post specifically about your situation, what you have done so far etc