2002 Chevrolet Silverado 2500 HD Question: what's causing steady vibration worse over 60mph

new tires helped some /allignment checked good /once every few months makes loud rumble from rear that lasts a few minutes then disappears . vibration is present even at lower speeds / can feel it even thru steering wheel and dash and floor / 55,000 miles .when going over 70mph , there is a deep humming sort of sound .