AT APRX 1200' MSL, WHILE ON FINAL APCH TO RWY 23R, THE PLT & PSGR NOTED A 'DIFFERENT' SOUND FM THE PROP/ENG. THERE WAS A LOSS OF THRUST FM THE ENG. ALSO, A ROUGH OR OUT-OF-BALANCE COND WAS NOTED. THE PLT STATED HE WAS UNABLE TO MAINT ALT & WAS FORCED TO MAKE AN EMERG LNDG ABOUT 3/4 MI SHORT OF THE RWY. DRG THE LNDG, THE ACFT ENCTRD SOFT/UNEVEN TRRN & NOSED DOWN. NO PREIMPACT MECHANICAL PROBLEM WAS FND WITH THE ENG. A POST-ACDNT EXAM REVEALED THAT ALL 3 PROP BLADES WERE FREE TO ROTATE 360 DEG IN THEIR HUBS. FATIGUE FRACTURES WERE FND ON 2 OF THE 3 PROP BLADE ACTUATING PINS; FATIGUE STRIATIONS WERE FND ON 90 TO 95 PERCENT OF THE #2 PROP ACTUATING PIN & ABOUT 60 PERCENT ON THE #1 PROP PIN (B-3491 PINS). NEITHER OF THESE PINS HAD BEEN REWORKED TO SHORTEN THE THREADED END IAW AD 77-13-17, WHICH REFERENCED MCCAULEY SERVICE BULLETIN #99. DRG INSTLN, THE PINS HAD 'BOTTOMED OUT' IN THEIR RESPECTIVE TAPPED HOLES, WHICH PREVENTED PROPER SEATING. THE #3 PROP BLADE ACTUATING PIN WAS A CORRECT PIN (B-4460) & IT HAD NO PREIMPACT CRACK OR FAILURE.