i have yet another question

since i have been on has had two transmissions but other than that it has been great. THe only thing i still dont know about is that the car shakes under load. when i driving and have it in gear it shakes side to side when i accel in all gears. when i push the clutch in it instantly gets smooth,put it in gear and accel it shakes again.I can put it in neutral and rev to 4thousand rpms and it is smooth. i think it has something to do with a tranny mount possibly. any ideas thanks again