THE PLT RPRTD THAT DRG DSCNT, AT ABOUT 150 KTS, TURBULENCE WAS ENCTRD & THE RUDDER PEDALS BEGAN SHAKING UNCONTROLLABLY ALONG WITH THE WHOLE ACFT, MAINLY THE EMPENNAGE. PULLING BACK ON THE CONTROL COLUMN & PUSHING ON THE RUDDER PEDALS CAUSED THE SHAKING TO SUBSIDE. A LANDING WAS MADE WITHOUT FURTHER INCIDENT. ALTHOUGH THE AFT FUSELAGE SKIN WAS WRINKLED, THE ACFT CONTINUED TO BE FLOWN FOR SVRL MONTHS. A SUBSEQUENT INSPN DISCLOSED THAT ALL 4 CORNERS OF THE AFT FUSELAGE BULKHEAD WERE CRACKED. THE STATIC BALANCE OF EACH RUDDERVATOR WAS MEASURED & FOUND TO BE EXCESSIVELY TAIL HEAVY. ALSO, BOTH RUDDERVATOR TRIM TABS HAD BEEN INSTALLED IMPROPERLY, RESULTING IN FREE PLAY AT THE HINGE LINE. NO LEAD BALANCING WASHERS WERE FOUND IN EITHER RUDDERVATOR COUNTERWEIGHT HOUSING. THE THICKNESS OF PAINT ON THE RUDDERVATORS EXCEEDED THE MANUFACTURER'S FACTORY ACCEPTANCE CRITERIA.