Hi Ultra,
My first suggestion is to put the rear shock pressure down to a lower setting and adjust the spring preload to see if that helps. Bearing replacement is only necessary if they "feel" bad upon inspection. Depending on the miles and care of maintenance that has been done to the bike would determine if an inspection is even necessary. And the vibration would really need to be isolated since it may not be being cause by the wheel bearings. Is the vibration coming from the rear, the front, the midway? Have tires been replaced recently or are due for replacement? Are the swingarm bearings in good shape? Is the differential and drive shaft in good condition? Getting the unit off the ground for these inspections can be done at home. Then begin a methodical exam of the unit front to rear. Even the steering head bearings can transmit vibrations through the frame and make the rear feel unstable.