Big time rumble and shaking when going 45 to 55 mph, then check engine light flicker on 2001 Ford F-350 Super Duty

When I am going 45 to 55 a rumble can be heard in front of truck and will end when tranny shifts. Truck will shake for a short distance. Also the check engine light will flicker on then go out

NEED to get that code for the Check Engine Light ASAP! Then you will have something tangible to trace and repair.