I don't know what to say about this problem, but my truck does it too. It's caused by the firing pulses of the engine that get amplified by the axle wrap of the leaf springs. It hits a resonant frequency above 45 mph and just feeds on itself. New shocks and bushings could help this problem, but it may never go away completely. Oversize/unbalanced tires make it worse yet. Make sure the driveshaft is balanced, and the tranny mount is good.