Is the sound only present when gas is applied or constant at the 40mph mark? That can help determine driveline issue vs everything else. I would check your flex disc for cracks and make sure that is in working order. Also the center support bearing and u joint can go bad and have some of the symptoms you describe. Has your rear diff ever been serviced? When you floor it and there is a rumble is that from a stop or anytime you're hard on the throttle? If you're at the 40mph mark and its rumbling and you hit the brakes what happens?

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