Kryckter

I have been experiencing a vibration for a long time now. The vibe is only at 3500-4500 RPM's and does it in neutral or and gear. It does not matter what speed I am at. I have all new plugs and wires, I dont believe it to be a miss. I have a new balancer and have tried rotating the hub over. The motor is rebuilt but I saw the balance sheets and they were proper. It did this before the rebuild. Im thinking clutch out of whack or bad input shaft or something. Any advice on where to look or what to look for. I also have a poly tranny mount