I don't know about the 737-300 specifically but on some aircraft antiskid does cause noticeable vibration. Although this is not part of the designed motion buffet model, nor is it on the list of buffets explicitly required, it is sometimes added as a local mod as a result of FAA, CAA or CASA "requests for improvement".
It might be a motion maintenance issue. The simple way to test would be to do an RTO with the anti-skid off and see if the vibration still occurs. If it doesn't, it has been added to simulate antiskid vibration as the sim techs say.