Originally Posted by legerwn

The PCM uses top dead center of #1 cylinder to watch for crank shaft acceleration and deceleration to be able to identify misfires and associate them with a specific cylinder. If it losses the top dead center position it cannot identify each cylinder firing and will mistake these events as misfires. GM says if any part of the rotating assembly is replaced, a crank variation relearn needs to be performed, this would include a new transmission and/or torque converter.

If the truck is running fine with no detectable misfires at lower speeds then I would suggest having your mechanic perform a crank variation relearn and see if that fixes it. The few times I've seen this happen the flashing CEL comes on around 55 MPH and above and will stop flashing and turn off the CEL after slowing down.

The crank variation relearn procedure will re-establish TDC in the PCM

Neal