1991 Ford Aerostar Van will not go into Park

I purchased a used car ('91 Ford Aerostar LX Van) for temporary A to B use while my other car is in the shop. Unfortunately, I don't know a lot about cars and don't have any mechanic friends, so I just did the best I could. The fluids were all good, no apparent leaks, no smoke, no funny smells or noises. When I took the car for a drive it ran great, shifted nicely (very smooth and no jerks), plenty of power. The owner told me the transmission was strong so far as he knew; never had any problems. So I bought the car and hoped for the best. The one problem I've noticed is that the transmission will not shift into the Park position. Actually, the shifter goes into place just fine, but the car is NOT parked. I have to use the Emergency Brake so that it won't roll. When I drove the van back to work, my boss made a comment, "Yikes, Park is what locks the transmission. Hope you don't have transmission problems." Now I'm a bit concerned. Do you know if this is a common problem that is easily fixed, or should I get rid of this thing like a hot potato while it still runs good? ;) I never suspected tranny trouble because it ran so great. Thanks for any advice you can offer. I appreciate it! :)