Limp in mode is no fun. I did not deal with it personally but a friend did and drove 25 miles that way.
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something is off your staying stuck in 6th gear @ 40mphSounds but little feel at the steering wheel.
Only thing I can cause to reproduce it is 40MPH at 1000RPM.
Where to from here?
No codes.
See if it coincides with AFM activation. Cycle through the DIC screens until you get to the one that shows "V4 or V8 MODE" and drive so that it shudders. Blip the throttle and see if it clears up along with being in V8 mode, then if it returns once back in V4 mode on deceleration back to 40 MPH.
You could also hold your trans in M4, accelerate above 40, then let it decelerate in 4th down to 40 MPH. If it doesn't shudder at 40, let it continue to decelerate until you're at 1,000 RPM and see if it shudders then.