A PART 135 ACR ACFT LEFT THE RWY; AFTER LNDG IN WINDSHEAR CONDITIONS; AND EXPERIENCING DIFFERENTIAL PWR.

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ON LNDG AT SLC; CLRED FOR ILS RWY 16; ABOUT 6 MI N; THERE WAS A CELL; THAT WE BARELY AVOIDED BY GETTING AN EARLY TURN. WE COULD SEE THE RWY AND THE TWR RPTED WINDSHEAR; WITH WINDS OUT OF THE S AND N; BOTH VALUES WERE UNDER 10 KTS. THE ATIS RPTED WINDS OUT OF THE SE AT 15 KTS WITH GUSTS TO 25 KTS. FO WAS FLYING AND LANDED LONG DUE TO TAILWIND; AND DUE TO OUR KEEPING THE SPD UP FOR WINDSHEAR. WE HAD PLENTY OF RWY; THOUGH. AS WE WENT THROUGH APPROX 70 KTS; I ASKED FOR THE ACFT NORMAL PROC IN THE SW3 (SWEARTINGEN MERLIN III) SINCE THE STEERING IS ON THE CAPT'S SIDE; AND ALL TAXIING IS DONE BY CAPT. I WAS IN THE PROCESS OF BRINGING SPD (LEVERS) BACK; AND GOING TO ARM NOSEWHEEL STEERING; WHEN THE ACFT TURNED TO THE R AND CAME TO A STOP; BTWN RWY AND TXWY. SHUT DOWN SYS AND EVACED PEOPLE. THE ACFT HAD A HISTORY OF MAKING THESE VENTURES TO THE R. I BELIEVE THAT ACFT PWR CTL LEVERS WERE MISRIGGED; WITH THE L PRODUCING A LOT MORE PWR AT GND IDLE. WE HAD NOTICED IT ON PRIOR LNDG; BUT HAD ASCRIBED CAUSE TO WINDS. IN HINDSIGHT; I SHOULD HAVE MADE THE LNDG; NOT DUE TO ANY INEXPERIENCE ON THE COPLT'S PART; BUT IN AWARENESS OF ACFT'S TENDENCY TO DART TO THE R; AND THE SHIFTING WINDS. I REALIZED THAT THE TRANSITION FROM COPLT TO CAPT WOULD HAVE BEEN TRICKY UNDER EVEN NORMAL CONDITIONS. NO INJURIES RESULTED TO PAX OR CREW. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THAT THE COMPANY REQUIRED A LINE- CHK; WHICH WAS COMPLETED SATISFACTORILY. THE ACFT PAPERWORK SEARCH; OF THE PREVIOUS YEAR; REVEALED BLOWN TIRES; AND OTHER SMALL PROBS; THAT WERE INDICATIVE OF MISRIGGING. THE EXCESSIVE TORQUE ON THE L ENG; WITH WINDS; LED TO DIFFICULTY IN GND TRACKING; ON THE RWY.