DC9-90 TRYING TO MAINTAIN SPACING BEHIND B737; FAILED TO CHANGE TO TWR AT THE OM. LANDED WITHOUT CLRNC. TWR ADVISED NO PROB AND HAD CLRED THEM TO LAND.

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WE CALLED THE PREVIOUS B737 IN SIGHT AND WERE TOLD HE WAS 3 MI AHEAD AND TO FLY 300 DEGS TO INTERCEPT RWY 27L LOC CLRED FOR THE VISUAL APCH; MAINTAIN 180 KTS TO THE MARKER AND CONTACT TWR 119.1 AT THE MARKER. WE ACKNOWLEDGED AND THEN BECAME VERY CONCERNED WITH THE SPACING BEHIND THE B737. HE TOO HAD BEEN ASSIGNED 180 KTS; BUT WE SEEMED TO BE GAINING ON HIM. WE FELT OBLIGATED TO MAINTAIN 180 KTS PLANNING TO THROW EVERYTHING OUT AND SLOW AS MUCH AS POSSIBLE WHEN WE GOT TO THE MARKER. WE BECAME DISTRACTED BY THIS CLOSE SPACING AND FORGOT TO SWITCH-OVER TO TWR AND LANDED WITHOUT RECEIVING CLRNC. LATER ON THE PHONE ATC ADVISED US THAT WE HAD BEEN CLRED TO LAND WE AND WOULD NOT BE VIOLATED. ALTHOUGH I ACCEPT FULL RESPONSIBILITY FOR MY ERROR; I FEEL THIS PROC OF BEING TOLD TO CONTACT TWR IN 5 MI IS FRAUGHT WITH OPPORTUNITY FOR SUCH ERRORS. THE APCH CTL FREQ WAS EXTREMELY BUSY AND WE COULDN'T GET A WORD IN EDGEWISE TO CONFIRM THE B737'S SPD. (ALMOST ALL FREQS THESE DAYS ARE TOO CONGESTED). ADDITIONALLY; THE FACT THAT THIS AIRLINE HAS CANCELED 36 CONSECUTIVE DAYS OF MY VACATION HAVING ME ON DUTY FOR 423 DAYS AS OF TODAY WITHOUT A VACATION CERTAINLY HAS NOT CONTRIBUTED TO THE SAFEST OF OPS.