This 'unique randomization' reminds me (don't laugh too hard) of APL.
It begs for the "?" operator (as I recall).  In unary mode it returned
a random number <= the argument; in binary mode it returned "x" unique
random numbers <= the second argument.  You could then accomplish the
entire dastardly deed by:

   fileEOF?fileEOF

and assigning the resulting vector to a variable.  You now have a
randomized list of record numbers.

How to do FREADDIR's in APL is left as an exercise for the reader :-)