It sounds like a shift linkage problem. Most likely the linkage is no longer attached to the shifter, or there's a problem at the transmission where it connects.
You'll need to have a technician diagnose this.
You may also want to have the transmission service done if you haven't already. Check your owner's manual for a maintenance schedule for this service.
〉 Answered on Jun 30th, 2008 by Lori Johnson, Owner and Instructor at Ladies Start Your Engines