I cannot get the ignition switch to return to the lock position with the trans in park.  I can still remove the key, but can't get it to lock which means i can start the car without the key

Normally, you shouldn't be able to remove the key unless the ign. cylinder is "locked"....normally...
Looks like one of two things here...either the key cylinder is worn out/damaged, or the linkage from the trans shifter to the steering column that operates the interlock is out of adjustment. This is the linkage that is supposed to keep you from turning the key to "lock" unless the shifter is in Park.
There is a cable that connects to the shifter linkage on the trans, and it goes up to a lever on the bottom of the steering column, in the engine compartment. The cable may be worn, or even disconnected. Check that first. THen, you may want to dis-connect the cable from the column, and move the lever all the way up, and see if the key will turn all the way to lock. If it dies, then possibly a cable adjustment is all you need for that. The adjustment can be either on the top side, or on the lower end, by the trans.

Still, if you can remove the key without it(the key cyl.) being locked, the cylinder should be replaced. If you can start it without the key, some low-life thief can too...

I had that problem recently. I had been messing under the hood and tripped a lever that is on the column next to the firewall.  This is some kind of lock out that goes to the transmission that has long been disconnected.  I believe that it should be in the up position.

Joel, you must type faster than I do.

Thank again - It seemed to have happened when I had the front end up on ramps cleaning off a lifetime of dirt, grease and oil.  The keys are pretty worn.  Sounds like a combination of both.

If the keys are worn, most likely the wafer tumblers in the lock cylinder are too.  The GM key is called a side-bar switch.  The Lock is 2 pieces, with the tumbler assembly rotating inside the case.  In other words, when all the wafer tumblers are lined up (due to proper key insertion) a bar on the side of the rotating assembly pulls in, out of a groove in the outer case, and allows the rotating assembly to rotate. 

It sounds to me that the wafer tumblers are worn.  Many times the key will smooth out and be able to be extracted however, when turned to the lock position, it won't release until the key is reinserted.  If it turns from the lock position without the key inserted, either the tumblers are worn, or the springs holding the lock bar are worn/broken. 

In either case, Joel is correct, replace the lock cylinder.  If you have the knock-out codes, (saved from the hole in the key when original) you can have GM order you one.  I would recommend finding a reputable locksmith and have him/her key a new cylinder to fit if you really want to keep it original.  Otherwise, since the key only fits the ignition, a new ignition no matter the keying will work.

If you don't know how to pull down a steering column, I would strongly recommend finding someone you know who had done it SUCCESSFULLY before, or find that reputable locksmith (trust me, all of them aren't) and have him/her R&R the column and switch.  There are tricks since you have the key, but it is challenging, mostly to get it back correctly and catch all the flying pieces when the springs let go

Good Luck!

BTW, I was a locksmith for a "few" years. 

What a wealth of information we have here!

There is an NOS ignition lock cylinder on e-bay right now - $25