If the service was done correctly, then the rear shock absorber relay arm and pivot points should have been dismantled and lubricated with grease.
If the bolts have not been tightened correctly, then I would suspect the pillion passenger would feel the vibrations first via the rear sub-frame (maybe, possibly,could be....)
Rear brake dismantled/cleaned/lubricated but not put back correctly/tightened?.
Rear wheel taken out? put back correctly and tightened correctly?