I have an 04 Trailblazer LS 4x4 that is all stock and around 170,000 miles. I have a sudden jerk/shudder anywhere from 45 to at least 75 MPH. I have checked the codes and the only ones are a coolant temp too low and secondary air injection system code. I thought it may have been due to old transmission fluid so I changed the filter and flushed by starting up the vehicle and leaving one of the cooler lines off until new fluid came out. This had very little impact on the shuddering. I tried the brake switch test and so far the shuddering stops when activating the brake switch. I have tried keeping the brake pressed and shifting into drive and lightly accelerating and no shuddering or surges are noticed. I am just looking for some guidance for next steps.