Have this problem in an 02 Focus. It appears to be happening on the shifter side of the interlock, as often shaking the shifter a few times will get it to let go.
Planning to pull the whole shifter out when I get around to doing other major work on the car, (needs a new head after launching the #2 plug enough times there are no threads left, though it's gone a few thousand miles with JB Weld holding the plug in, I know that can't last forever and the next failure might not be so easy to fix) so I'll follow up if I can figure out what exactly is sticking.