I just spoke to the local MB mechanic, and he thinks it's either the tumbler or the steering lock. I've also read on this site about bushings wearing out, but I would think that if this was the cause, that I would have eventually been able to get the key out by wiggling the shifter, which I tried to no avail.

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87 300D - Running on Veggie oil; 260,000 plus miles; Original #14 head