If the key will not turn, then the problem is with the tumbler assembly not the column lock. The symptoms of a column lock problem is the key turns but the column remains locked in place.

I've seen several reports here of a vibrating sander, or similar, helps to get the key to turn and unlock. Believe me, that collar is made of some tough metal and probably a diamond bit in a dremel is going to take a long time, if you can even get access to it.

__________________
Sam

84 300SD 350K+ miles ( Blue Belle )