Here is some good info on the P0420 trouble code: http://repairpal.com/OBD-Code-P0420
I cannot speak to other code for the shift solenoid (P0761), I don't have any experience with it, hopefully someone else does.
The transmission code is for a solenoid failure in the transmission. The transmission pan is removable and a the solenoid I believe can be replaced without removing the transmission, a transmission shop should be able to help (change trans fluid and filter at this time). Solenoid should be easy to test. You could chose to erase codes and see if the transmission code comes back. I just read the link on P0420 code good information there. If higher mileage perhaps cat is bad.