4.3 vibration

ive got a 1991 gmc 4wd with a 4.3 v6 auto tranny there is a vibration when i rev up the motor the 4.3 has 143.000 miles on it the 4.3 runs great just the vibration is driveing me up the wall. been told that the 4.3 is bad for the flywheel cracking in between the bolts havent taken it apart to look and see if this is the reason for the vibration. does anyone know any other reason this could be vibrating and i looked at the damper on the front didnt see anything wrong with it . thanks for your help:banghead: