INJURED WORKER & OTHERS WERE REPLACING A TAIL PULLEY ON A BELT CONVEYOR. A CO-WORKER LIFTED THE TAIL PULLEY WITH THE FORKLIFT AND DROVE THE FORKLIFT INTO POSITION & LOWERED THE PULLEY. INJURED WORKER WAS HOLDING THE PULLEY WHILE FORKLIFT WAS BACKED OFF TO PREVENT IT FROM SLIDING BACK. WEIGHT OF PULLEY SHIFTED; PULLEY ROLLED AND CAUGHT FINGER.