833 OD vibration

I was reading through AJ's suggestions. particularly (C). those are some things I haven't really tried. the vibration happens from 75 to 90 mph (2500 to 3000 rpm in OD). so I was thinking I would take it up to 90, then put in neutral. I figure if the vibration is still there it has to be after the trans, but if it goes away then it must in the trans.