In addition to new mounts, disks, and center bearing you should probably take the shaft in to a drive line shop and have the ujoint replaced / shaft balanced. This is generally not a DYI due to the pressed flanges / yolks vs clip retainers. Although some here have managed. But the shaft still needs to be rebalanced using special equipment.

Even a small undetectable (manually) amount of wear in the ujoint will cause your very description (vibration). Even when my drive shaft was out I could not detect any "notches" or slop in the ujoint. And I've personally R&R'd many a ujoint over the years. Apparently it needed some torque to get offset.

Prior to having the shaft ujoint replaced I replaced the motor mounts, trans mount, flex disks, center bearing and carrier, differential mounts only to still have the same vibration at 35mph to 55mph.

Drive shaft rebuild cost me about $265. Now the car runs smooth as silk.

I'm betting the true source of your vibration is a slightly worn ujoint. That said probably caused by worn engine mounts, trans mount, center bearing / mount, and or flex disk etc.

Don’t forget to do the “roll back and forth” procedure mentioned above prior to torqueing up the trans mount, motor mounts, and center bearing mount.

P.S my car is a W126 SDL

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1986 300SDL 440,xxx