I am not a mechanic, simply a person who had a similar problem.
I think that the part that the key goes into is called the "ignition switch mechanism." I have not replaced mine, since all I have to do is turn it a little towards me, until the electrical system turns on and that releases the gear shift enabling me to go into Drive (or any other gear).
A new unit goes for approx $70, and I don't think it's too hard to put in, but since I'm not sure and am unwilling to go to my $120/hr Volvo mechanic, I will just continue to start the car the way I mentioned.
If you say the car drives fine, how do you get it out of Park?