COMMUTER LTT ENCOUNTERED ICING CONDITIONS AT CRUISE ALT AT NIGHT WITHOUT ANTI-ICE EQUIP OPERATING. ACFT DEVELOPED A VIBRATION AND THEY LOST 1000 FT BEFORE REGAINING CTL OF THE ACFT.

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ACFT WAS IN LEVEL CRUISE FLT WITH AUTOPLT ENGAGED; AT FL190. CAPT AND I BOTH FELT A VIBRATION THROUGH THE FLOORBOARDS. WE BOTH SLID OUR SEAT FORWARD AND CAPT TURNED ON THE INSPECTION LIGHTS. WINGS; PROP SPINNER; AND ENG INLETS DID NOT APPEAR TO HAVE EXCESSIVE AMOUNTS OF ICE. ABOUT 30 SECONDS AFTER FIRST VIBRATION; THE STICK SHAKER FIRED AND CAPT TOOK CTL OF ACFT. CAPT CALLED FOR ALL ANTI-ICE EQUIP ON. I CALLED MEM CENTER TO TELL THEM WE WERE DSNDING. THE ACFT DID NOT RESPOND TO RUDDER/ELEVATOR INPUTS. WAS IN A R BANK; NOSE DOWN AT ABOUT 1000 FPM. CENTER CLRED US TO A LOWER ALT AT ABOUT 16000 FT WE REGAINED CTL OF ACFT AND PROCEEDED TO A VFR LNDG AT FSM. WE BELIEVED WE ENCOUNTERED A FREAK PATCH OF SEVERE ICING AND ICED UP TO T TAIL ON THE ACFT; STALLING THE T TAIL. MEM CENTER DID NOT APPEAR INTERESTED IN OUR RPT OF SEVERE ICE OR OUR MESSAGE THAT WE WERE COMING DOWN! CONTRIBUTING FACTORS: LATE AT NIGHT; FATIGUE. AN EXAMPLE OF AN ARPT 135 SCHEDULE 'LEGAL BUT NOT SAFE.' OUR FIRST REACTION WAS EITHER A STRUCTURAL PROBLEM OR A PROP OVERSPD. WE LOST VALUABLE TIME BEFORE WE FIGURED IT OUT. WHEN WE TURNED ON THE DE-ICE BOOTS; WE GOT A FAILURE LIGHT ON THE RUDDER AND ELEVATOR BOOTS.