THE ACFT WAS LESS THAN 2 MILES FROM THE ARPT OF INTENDED LANDING DURING THE APPROACH WHEN THE CREW EXPERIENCED A LOSS OFELEVATOR CONTROL. THE ACFT STARTED DIVING TOWARD THE GROUND WHEN THE POWER WAS REDUCED. THE PLT APPLIED FULL POWER TO TRY AND RESTORE LEVEL FLT. THE CREW DID NOT RECALL USING THE ELEVATOR TRIM TO RAISE THE ACFT NOSE. THE APPLICATION OF POWER ACCELERATED THE RAISING OF THE NOSE WHICH WOULD HAVE RAISED AT A SLOWER RATE USING ONLY THE ACCELERATION OF THE DIVE. THE CAPT COULD NOT REACH THE ARPT RWY BUT WAS ABLE TO ALIGN WITH AND LAND ON A HIWAY. THE NOSE WAS STILL DOWN AND THE ACFT DESCENDING WITH THE AIRSPEED AT 120 KTS WHEN THE ACFT CONTACTED THE HIWAY AND CARTWHEELED INTO A SNOWBANK ALONGSIDE THE ROAD. AN EXAM OF THE ELEVATOR CONTROL REVEALED A PUSH-PULL ROD HAD BECOME DISCONNECTED FROM AN ELEVATOR TORQUE TUBE. THE CONTROL HAD BEEN DISCONNECTED WHEN THE ACFT WAS PAINTED IN 12/81 & WAS RECONNECTED WITH A NONSTANDARD, UNSECURED BOLT. MAINT RECORDS WERE SKETCHY AS TO THE PROCEDURES DURING THIS MAINT. FAA SURVEILLANCE DID'NT DETECT PRBLM.