Car stalls when put in gear.

Engine Mechanical problem 6 cyl Front Wheel Drive Automatic 102000 miles

Just bought a 93 Buick Century for my son. The car looked and ran great at the lot and also on a fairly lengthy test drive. There are no check engine lights, does not run hot, all liquid levels are good and they do not appear to be that old or broken down, no real funny noises, and runs like a champ on the highway. On the way home it began to stall at the toll booth. As soon as you put it in neutral it starts right up, idles great and then dies when its put in gear. Also, as you are slowing to a stop there is a loud grinding type noise then it stalls. I was wondering if it could be the transmission lock plug? Or, what other god awful thing could be wrong with it. Also if it is the plug, is there any harm running it with it disconnected. Someone told me it would use a little more gas but would run fine. If necessary to disconnect it is that something I could do myself because I have no clue where it is. Help.