SUPER MD80 FLC DISCOVERED AFTER LNDG THAT THEY HAD NOT RETUNED THEIR RADIO TO TWR FREQ FOR OBTAINING LNDG CLRNC.

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RWY 35C; 2 MI FINAL. ON ARR; ASKED AND RECEIVED SWITCH TO RWY 35C FROM W SIDE CLRNC VISUAL BEHIND ACFT 2-5 MI AHEAD. HAD TO SLOW BY GOING ABOVE GS TO GET 3 MI SEPARATION. WAS CONCERNED ABOUT ACFT COMING UNDER E TO W TO THE W RWY; AND SEPARATION AHEAD. EVERYTHING SEEMED NORMAL. FO STATED WE HAD LNDG CLRNC AND I QUESTIONED HIM AS I DIDN'T REMEMBER IT. HE CONFIRMED AND I THOUGHT ABOUT ASKING TO CLARIFY. BUT 'TWR' WAS VERY BUSY; SO I ACCEPTED MY FO'S STATEMENT (HE IS VERY STRONG -- SOMETIMES SLIGHTLY HEADSTRONG IN POSITIVE WAY). AFTER LNDG; WHEN HE WENT TO SET IN ALTERNATE FREQ TO NEXT GND FREQ; HE REALIZED WE WERE STILL ON APCH. TWR CLRED US HOLD SHORT OF RWY 35L WITHOUT COMMENT. DON'T KNOW WHAT THEY KNEW OR REALIZED. FACTORS: DISTR; END OF LONG DAY OF LONG TRIP; NIGHT; PREVIOUS LEGS ALL HAD TURB; WX; ETC; TFC; RWY SWITCH; BUT CLRLY I HAD A CHANCE TO CATCH THIS WHEN I DIDN'T REMEMBER THE CLRNC AND CONSIDERED CONFIRMING WITH TWR. I VIOLATED MY OWN RULE OF CHKING AND RECHKING TO THE POINT THAT I FEEL SOMETIMES I AM ANAL OR A JERK ABOUT IT; BUT ATC IS SO BUSY AND FAST IN XMISSIONS I HAVE BEGUN ASSUMING AGAIN AND THIS HAPPENED.