4l80e hard shift after 65 MPH

I have a 98 GMC Sierra K3500 Dually with the 6.5 diesel. After I go over 65 MPH it sets code PO7030, incorrect gear ratio, and thats when the hard shift starts. My scanner shows torque converter slip of 200 rpm. I should also mention, I have noticed no engine braking, when cruising down the freeway if I let off, the engine will drop to idle. I have replaced the speed sensors on the back of the tranny and the transfer case, but not the one inside the trans. Also if I run at around 75 my speedo is erratic and the trans slips to about 3 grand once in a while. I'm affraid it's the torque converter, any other suggestions