I have a 1992 chevy silverado 4x4 w/ a 4 inch body lift and 4 inch suspension lift.?

Between 25mph & 35mph the truck rocks violently side to side. It doesn't swerve or lose control it ROCKS. I've changed the bushings, replaced the idler ar, rotated and balanced the tires. If I mash the gas pedal or slow down to below 25, it stops. When I switch into 4x4 mode, it reduces the rocking so it's barely noticeable but I can still feel it. I'm stumped... Anyone got any ideas?