I didn't pass 40mp but i ran it up to 40 again on the lift today and it seems to howl more around 1500-2000rpm which is about 40mph in 4th gear on the lift, but it does it mostly in that rpm range. Although i haven't really gone any higher than that.
How loud is it though. Mine is disturbingly loud. The diff has been on bruces bushings for a while and this noise is just telling me that something is really wrong.