I had some similar problems last week. My problem was key was not going in fully. I removed collar and tumbler and sprayed with brake cleaner and shook it off then used about 10 drops of 3 in 1 oil. Look at yours and make sure the key is going in fully. It should bottom out at ledge on key. Roy told me the end of key touches something and unlocks the steering lock mechanism. Maybe you key is not going in all the way.

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1982 300SD