I had to look it up because I'm bored at work and that number didn't make any sense.
His last postseason RBI with w/ runners in scoring position was in Game 2 of the LDS in 2004 (Bottom 12th, double scored Miguel Cairo from 2B). He did have 2 RBI in Game 3 of the 2004 LCS in which Jeter scored from 1B twice but they obviously don't count.
The 57 is definitely not for runners in scoring position and his active streak is only at 15. You can check out box scores here...
http://www.baseball-reference.com/postseason/