I'm looking for ideas!

About a year ago, I started to notice a slight vibration drone at speeds above 40mph. Now it has become pretty persistent and I can't seem to find the source. As speed increases, the droning noise diminishes and pretty much is non-existent about 75-80mph and I notice it most with harder acceleration.

Currently,
Different driveshaft
new Carrier Bearing and mount
new flex disc
new front tires
rear tires balance checked

The only thing I haven't changed in a while is the transmission mount but it was replaced about 5 years ago. Has anyone else encountered a transmission mount causing this kind of problem?

I've raised the car and ran it up to higher rpm while checking the driveshaft but can't see any vibration of components. Of cours this is without a load on the drivetrain so I'm not sure of the validity of checking things this way.

I would like to send the driveshaft out for rebalancing, etc. but it's fairly expensive and would hate to go to that expense to only result in no change.

Could an alignment problem cause this? I believe it's been 2-3 years since last done.

Thanks for any insight on what I haven't tried.

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Sam

84 300SD 350K+ miles ( Blue Belle )