Problem solved! Took it to the transmission shop yesterday. He originally did not suspect the torque converter as the problem was occurring between 20 & 30 mph, vs at higher speeds. He was able to replicate the shuddering, where no codes were found and the torque converter pressure was normal. However, he stated that he and his tech were 70-80% certain it was the converter. He recommended adding two bottles of Instant Shudder Fixx and if this fixed the problem then no further repairs would be recommended. I have attempted several times to replicate the shuddering and I could not. So appears to be the torque converter, but with an unusual condition.