4.6 vibration at 2500+ RPM

[Thorwon]

I've built a 4.6L short rod stroker. I used a 1988 Comanche block, the light crankshaft and rods from a 4.2L.

SPEED PRO Hyper H802CP40 pistons.

The camshaft is a Crane Cam H-260-2, .456 intake .484 exhaust.

And a 7120 head.

I put a new Dorman 594-018 Harmonic Balancer on it.
It starts and runs great, but at about 2500 RPM there's a vibration and the higher the RPM the worse it gets.

Idling it's as smooth as it should be.
Everything I can think of is right. 
Anyone have any ideas what it could be?