Vibration at 3200 rpm and above need help

When it was tested on the jackstands, did you perchance remove the wheels and drums, to rule them out?

In my 68 S-clone,I've run 4.10s and 4.30s, both of which excited the natural body vibration. They both made that characteristic quiet "whoop-whoop-whoop"sound throughout the car and both at 55 to 60 mph.This was not a vibration. No other gear ratio that I have run ever did this(2.76 to 3.55,3.91,4.89 and 5.13s) These were all just chunk swaps.The 4.3s were bearable but the 4.10s I sold to a guy with an E-body and problem solved. I had a 70 Swinger340 that did it too.
I think As might be allergic to 4.10s! LOL