Weird Vibration!

I am not a tec so I am sure some one will jump in here and help you along.

Do you know how a house floor fan runs smooth at low or med, then put it on high and she gets to shaking.:-D
I think when the trans is in neutral the rear tires are spinning your gears and pinion and up tough the drive shaft to the tail shaft of the trans.
I am sure you can have the drive shaft checked for out of balance easy and at a low cost.
I think I would put it up on stands and have someone get the running gear up to 30 or more and look at the area's that could be vibrating.
Remember I am not a tec:stop: so be careful and hope you get this figured out.

Just me, old shade tree man.:read2: