RPTR FOLLOWED THEIR TCASII RA TO AVOID OTHER TFC.

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UPON LEVELING AT 4000 FT AND HDG E WHILE BEING VECTORED FOR RWY 17C; WE RECEIVED AN ALT ALERT ON THE TCASII. SHORTLY THEREAFTER; APCH POINTED OUT THE TFC TO US. WE THEN GOT A TCASII COMMAND TO 'DSND XING TFC.' WE BEGAN A DSCNT AND NOTIFIED APCH. THEY TOLD US NOT TO DSND BECAUSE THERE WAS TFC BELOW US AND TO CLB TO 5000 FT. WE NOTIFIED THEM THAT WE COULD NOT CLB DUE TO A TCASII RA. THEY THEN TOLD US TO CLB BACK TO 5000 FT AS SOON AS WE COULD. WE DID NOT HAVE ANY TFC ON TCASII BELOW US; BUT DID HAVE THE TFC ON TCASII THAT WE WERE REACTING TO. WE FELT WE WERE IN A NO WIN POS AND TRAPPED BTWN 2 OTHER ACFT. ONE WE KNEW ABOUT ON TCASII; AND THE OTHER WE HAD NO IDEA WHERE IT WAS. WE DSNDED TO 3500 FT AND THEN CLBED BACK UP TO ALT. AS FAR AS WE KNOW; THERE WAS NO LOSS OF SEPARATION. IT WAS A VERY UNCOMFORTABLE POS. THE CTLR; AS SOON AS HE SAW WHAT WAS HAPPENING; TRIED TO CORRECT THE SIT; BUT WE WERE ALREADY WELL INTO OUR DSCNT AND WERE AFRAID TO GO AGAINST THE TCASII AT THAT POINT. I WOULD STRESS THE IMPORTANCE OF FOLLOWING TCASII. I BELIEVE IT KEPT US OUT OF A SERIOUS SIT.