Boy that sounds odd.

When you are in gear you are loading engine, trans and driveline. When in neutral, all you have is driveine and part of the trans.

Can you see if you can duplicate the vibration WHILE IN GEAR by carefully applying small degrees of accel or decell?? Does braking impact this at all? light brake?

A bit of a WAG, but Id look at engine and trans mounts and all thrust arm bushings.

The very tricky part is that we KNOW that visual inspection of BMW bushings is not fool proof- there are common bushings that fail, causing odd vibrations, but look AOK.

HTH