2011 Cascadia w/DD15 - getting check engine light over 65 mph

I recently bought a used 2011 Cascadia with the DD15 motor, no DEF. Anyway, I'm getting a check engine light anytime I accelerate over 65 MPH. Has anyone ever had this or have any idea of what it might be? Truck is running good, no engine performance problems even when light is on. As soon as I back off to 65 mph or less, the light goes out.

 

Thanks mhyn. Had them check and "speed limit" as well as "cruise limit" were already set to 85 mph. However, there was a third speed parameter (not sure which one) that was set to 65 mph. Increased it to 85 mph and problem solved! Thanks again!