THE PLT WAS PARTICIPATING IN A CAFE-400 (COMPARATIVE ACFT FLT EFFICIENCY) CONTEST OVER A 400 MI, ROUND ROBIN COURSE. WHILE DSCNDG FM 7000' AT THE END OF THE FLT, THE PLT SLOWED THE ACFT TO 140 MPH TO OBSERVE TRAFFIC. WHEN CLEAR, HE INCREASED THE SPEED TO 160 MPH & BGN A 500 FPM DSCNT. AT ABOUT THAT TIME, HE DETECTED A VIBRATION THAT INCREASED IN INTENSITY, THEN NOTICED THE SPEED HAD INCREASED TO 188 MPH. THE PLT REDUCED PWR, & AS THE ACFT SLOWED, THE VIBRATION STOPPED. THE ACFT WAS LNDD WITHOUT FURTHER INCIDENT. A POST FLT EXAM REVEALED THE AFT FUSELAGE WAS WRINKLED & BUCKLED IN A 3 FT AREA, JUST FWD OF THE MAIN SPAR BULKHEAD FOR THE V-TAIL. THERE WAS EVIDENCE OF FLUTTER WHICH HAD RESULTED IN DYNAMIC OVERLOAD. AN INV REVEALED THE RUDDERVATORS WERE OUT OF BALANCE. THE ACFT WAS BEING OPERATED NR ITS MAX GROSS WT LIMIT WITH AN AFT LOADING, BUT IT WAS WITHIN THE WT & BALANCE LIMITS.