I'm not familiar with those years, but if it gets locked up, then your shift lock may be dying. You know, that lock that disengadges the second you hit the brake pedal to release the shifter into gears. It's more of a safety feature that prevent the shifter to be shifted if the brakes aren't applied.
This is my best guess. By how you described, sounds like it doesn't happen all the time, do you apply the brakes every time U shift from park to any other gear???.