I've been chasing a similar vibration. Swapped axle buckets and tried 2 different sets of wheels and tires to no avail. Mine gets real noticable from 65mph to 75 mph. Sustained or under acceleration. I'm gonna try to replace the entire axle and see what happens. Otherwise, I'm getting stumped as well.

 

my pinion nut got loose and caused a vibration at 65mph.

tightened it up and no more vibration.