so are you getting vibrations at highway speeds now? if so it could be bad bushings. i run 63" chevys with a standard shackle and a lift block and mine rides like a caddy. personally i'm not a fan of the double shackle, yes it flexes good but their many bad characteristics too. single shackle and the 63s, if set up right will flex just fine.
mike
vibrations at all speeds but worse at highway speeds..
