2004 Chevy Silverado Shaking and pulsing while driving

I have a 2004 Silverado extracab w/ 52000. Recently, after the vehicle has warmed up, it shakes/pulses (like going over a rumble strip) between 40 and 55 mph. It goes away if let off the gas or give it the gas to accelerate. Acceleration is fine, I even punched it from a start to 60mph no problem. In park I put it thru the rpm range with no problem. The engine light is not on/its not sending codes and my mechanic says its not rotors or suspension and is stumped!
Help! Thanks.