The lock itself probably isn't the problem, but if you want to replace it to be able to have the key lock function, that would be worthwhile. I suspect that the ignition switch itself has failed or is out of adjustment. It is actually lower on the column than the ignition lock. It is topside below the dash, held on by two screws.

From Autozone.com:
With Key

1. Disconnect the battery ground.

2. On tilt columns, remove the upper extension shroud by unsnapping the shroud from the retaining clip at the 9 o'clock position.

3. Remove the trim shroud halves.

4. Unplug the wire connector at the key warning switch.

5. Place the shift lever in PARK and turn the key to ON.

6. Place a 1 / 8 in. wire pin in the hole in the casting surrounding the lock cylinder and depress the retaining pin while pulling out on the cylinder.

7. When installing the cylinder, turn the lock cylinder to the RUN position and depress the retaining pin, then insert the lock cylinder into its housing in the flange casting. Assure that the cylinder is fully seated and aligned in the interlocking washer before turning the key to the OFF position. This will allow the cylinder retaining pin to extend into the cylinder cast housing hole.

8. The remainder of installation is the reverse of removal.