Transmission problems

:disappoin 1991 buick park avenue

transmission problems: We bought the buick used and had much good luck with it up until 6 months ago. I drove it home and it felt as though the brakes were dragging when I pulled it in the garage. The engine light came on. I check the codes for the problem and got a number of engine errors: code 16,21,24 were the first three. I then took it to the local dealer. He checked it out and said the transmission was bad among other unspecified problems and the car would cost more to fix than it was worth. Took it for a second opinion and they said definately it was the transmission. They also said that the torque converter was locked up. The car is also running hot since this all began. It is an excelent looking car without rust anywhere on it and I'd like to fix it if possible.

My question is can all these problems be caused by the transmission? Any help from a professional would be appreciated.

Thanks

I need to add to this post that the car wasn't overheating initially. It began overheating after it sat for a few weeks. Also I inspected the drivebelt for the waterpump and it is up to specifications. Before I do anything with the transmission I plan to have the radiator checked out to see if it is partially blocked. How difficult is it to replace the transmission if I do it myself? I used to do quite a bit of mechanic work, but now I don't have the extra time that I used to have plus with all the advanced technology on autos these days I'm not sure I could handle the job. Any help or advice would be appreciated.