Chasing vibration...help!!

So I'm chasing a surging type drive line vibration. 75 Duster 360/727/ 8.75 3.23 open 489 case. Let me start by saying...trans is rebuilt, new converter, had driveshaft made and balanced, new tires...yrs they are balanced...I even pulled the extension housing off the trans last week and put a new bushing in just to try and eliminate that. Still there....the only thing I haven't messed with is the rearend...I'm 46 and still have all my hair but if this **** keeps up I'm gonna start pulling it out..lol. This vibration starts about 35 to 40 mph and gets a little stronger as you go up in speed. It surges in and out of vibration....What am I missing?