In my 1984 300D, it wasn't the tumbler but rather a "spring detent post" in the innards which was the problem. You must remove the steel cover plate on the key stalk to access it. The stalk must be out of the car for this, which is a bit tricky since you must pull the dash hole forward a bit to swing it clear, plus push a hard-to-see release button to pull it out. You can only push that release in when the key is in a certain position.

This spring post is thin die-cast zinc or such. It breaks off, then jams the rotating mechanism. It seems so wimpy that I am surprised it hasn't broken off in more cars. Mine no longer has that spring detent, so the key doesn't click into positions as solidly, but hasn't ever rotated while driving. Of course, don't hang heavy junk off your key or you will have problems, in many cars especially recent Chevy's.

I should have removed the steering lock rod while I had the stalk out and that cover off, since I doubt the anti-theft is worth the risk of jamming the steering wheel. I debated that, but left it in to be "correct". I staked & epoxied the steel cover plate back. Good luck.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans