If your convection fan motor is shaking badly you either have a dirty fan blade, faulty rubber bushing, or bad motor bearings.
If the shaft of the motor is moving up and down at the end cap of the motor then you have bad bearings in the motor.
If the shaft is not moving up and down at the end cap, and the motor is vibrating you have a bad bushing.
The old bushing should just pop off after removing the clamp, and the new bushing should just pop back on. Make sure to put the clamp back on the new bushing.
If you are having trouble putting the new bushing on you can cut the bushing to install then put the clamp on, and it should work just fine.
*NOTE*
All Leeson motors we use have the same rubber bushing part# 523001