there is a chassis stabilizer kit to fix that problem due the engine/trans. being rubber mounted.im sure others will help out with more info. . I have not had this issue but heard quite a bit about it and have seen that kit online for it
I'd recommend the Glide Pro Stabilizing System (link below). This System replaces the rear bushsings, shaft and motor mounts. Most importantly, it will completely eliminate the high-speed wobble that you describe.
