THE PILOT HAD JUST COMPLETED A CLIMBOUT AND WAS ADJUSTING THE TRIM FOR LEVEL FLIGHT, WHEN THE AIRPLANE BEGAN VIBRATING. HE LANDED IMMEDIATELY. EXAMINATION SHOWED THAT A PORTION OF THE ELEVATOR SERVO TAB HAD SEPARATED. EXAMINATION OF THE REMAINING SECTION OF THE SERVO TAB SHOWED THAT THERE WERE CRACKS IN THE TRAILING EDGE. THE DIRECTOR OF MAINTENANCE REPORTED THAT WHEN THE AIRCRAFT WAS PARKED, THE TAIL WAS ABOUT 10 TO 11 FEET ABOVE THE GROUND AND WOULD HAVE BEEN DIFFICULT TO INSPECT DURING PREFLIGHT.