Its gotta be something that is effected when only under load. Its most apparent under HEAVY load correct? If you ease it up to 50mph at say 25% throttle, its not evident correct?
Are all your axles new? Ive gotten reman ones from the autoparts store and they shook as your describing worse than the broken ones. I think your problem lies in the CV joints having to much play/clearance. Know anyone that you could borrow their axles to see if it makes a difference?