I finally got the repaired TCU in the car today. I actually sent it out to David Ruberg (http://www.speedometerrepairguy.com/) to have my old one fixed as I had trouble sourcing a replacement. I reinstalled it today and so far so good! The D4 light is off and the car dives great. Big thanks to College Man for your help. Hopefully others will find this thread useful in the future!