Recursion works by reducing a problem to a simpler problem of the same kind. What you need to do is reduce intDiv(n1, n2) to intDiv(n1-n2, n2), and terminate as appropriate.
Is it the fact that two numbers are involved that confuses you? That should be familiar if you have programmed Fibonacci recursively. Another example you could study is Euclid's GCD algorithm. [ January 01, 2006: Message edited by: Ulf Dittmer ]