Vibrations in drivetrain

As I have posted several times in the past, I have a vibration I can't find. I have changed tires, wheels, had the 904 rebuilt with new tailshaft bushing, had a brand new driveshaft built and balanced. I was told by the guy I bought the engine from that is was indeed balanced. I was also told it was an internally balanced engine and came with a neutral dampner. I am using a torque converter with the weights knocked off. What kind of vibrations would one have if it was an externally balanced engine with neutral torque converter and harmonic balancer?? This thing makes me want to get rid of it!!!!!!!!!

I forgot to mention, I also tried a different 3rd member.

what size of engine is it?

Have you ever check the rear pinion angle?

It's a 340, I haven't checked the rear pinion angle but it looks like it is about parallel with the drive shaft on the ground. I have played with air shocks on the rear in different positions and doesn't change anything at all.

You mention you changed the third member. Did you check your axles to see if one or both aren't bent or twisted? Bearings? When the car is at ride height is the driveshaft not jammed to tight into the tail stock on the trans?

I had the car on my lift with the axles out and at ride height. The driveshaft I thought was too short so I had a new one made with about 3/4" to go before it would jam.

The axles were not bent or did you check them? Maybe try the old driveshaft and see if that makes a difference.

I did not check the axles because I ran the car up to speed on my lift with the axles removed but still had the 3rd member in and it still vibrated. The vibration is there with the old or the new shaft installed.

the vibrations changed when I changed gear ratios. But it was the speed where they occurred that changed not rpm.

Well, it looks like you have covered the bases. I think AARCuda might have it on the pinion angle. I believe it should be set to 5-7 degrees down if I'm not mistaken.

eng balance/TC prob

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the vibrations changed when I changed gear ratios. But it was the speed where they occurred that changed not rpm.

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If its is Speed related then it must be in the diff, but if it is RPM related its in the driveline. You knocked the weights off the external balanced convertor..some of those were to balance it neutrally too. It would show itself in neutral too.