If the chatter is at low rpm's (600-900 rpm) the clutch/flywheel balance is most likely not your problem. The imbalance would show up more and more as you increased engine speed. Check your motor mounts again, u-joints and rear end pinion slop. If you have a lot of looseness in the rear pinion mesh it is likely to cause clutch chatter. Also check for loose motor to bell housing bolts and transmission to bell housing bolts. Another possibility is bad rear spring rubber bushings or loose u-bolts. One final problem could be improper clutch finger adjustments. To check this, put the vehicle in neutral, set the hand brake, chock the wheels to prevent rolling, start the engine and let it run at idle. Slowly depress the clutch pedal and note if there is a pulsation in the pedal. If there is a noticeable pedal pulsation, the clutch pressure plate needs replacement or adjustment. This last test eliminates motor mounts, u-joints, rear pinion, rear spring bushings, loose rear spring u-bolts and flywheel/clutch pressure plate/clutch disk balance problems.