Just to be very clear, is the vibration on rotate, or 8-10 seconds after rotate?
If it is on rotate, it cant be the gear. You got to remember there are a few wheels to retract on a 747, it takes a little while!!
If the vibration occurs after the rotation and airbourne, it is more than likely the gear has been retracted and wheels are still spinning giving the gyroscopic effect in the undercarriage bay.
It's good that you brought it to the attention of the crew - good CRM!!