Just Replaced Transmission Starts In 2nd Or 3rd

Just finished replacing the AOD trans in a 2006 v6. It won't shift any forward gears. Starts off in 2nd or 3rd. Manual shifts to lower gears dont have any effect. Same result. The original box lost reverse and I purchased a used trans online. (maybe a big mistake?) Now I have a CEL, flashing O/D light and no gear changing. Oh, reverse works. Any thoughts? I don't relish the idea of yanking that trans again. What a PITA!
I have a code reader, but it is not available at the moment.

Update! got 4 codes P0743 torque converter clutch solenoid circuit, P0750 Shift solenoid A, P0755 Shift Solenoid B, P0760 Shift Solenoid C, P0765 Shift Solenoid D. gee, looks like all my shift solenoids are hosed. Perhaps there is another answer? Help!

2nd update. Found that big plug that bolts into the socket on the left side of the trans was loose. tightened it, but the same codes and same behavior still exist. I hate this car.

3rd update! (this writing my own solutions/replies is getting old...you guys can weigh in any time you want)
Jacked the car up, set the jackstands,and dropped the back x-member to lower the whole package. I thought maybe, just maybe, that big plug with the bolt in it might still be the problem, but the only way to really get access to it was to lower the trans. Which I did. even then, I could only get an end wrench on it,an only get maybe 1/6th of a turn. It took a lot of 1/6th turns. a whole lot. I'm getting pretty good at navigating the underbelly of that mustang. I feel like a Ford Proctologist.
Anyhow, I got the plug fully seated, and the gearshifts came back. The thing is still running rougher than a cob when in gear at an idle, but goes like stink when you flatten the loud pedal. The one problem that persists (other than the rough idle) is the O/D light is still flashing. I cannot reengage the O/D nor can I make that light go out. Any ideas?
Oh, btw, I cleared the 5 codes, and none have come back (yet...)