The vibzzz are killin me!

Maybe you guys can help me with this vibration and cannot seem to find. I have a 67 barracuda with a 410 stroker, 727, 8-3/4 rear with 2.94 gears and 17" wheels. I have a vibe at approx 60-70 mph@2600-3000 rpm. I recently rebuilt the engine so Im thinkin oh crap. It does not vibe in park or neutral at those rpm ranges, so i figure im cool. I had the tires balanced, no go. I replaced the iso driveshaft with new and u-joints, no go. I put the rear on jack stands and ran it and can feel the vibe. I pulled the wheels and its still there. Pulled the brake drums and its still there. I pulled an axle from one side and still there. I pulled the axle from the other side and its still there. I figure okay I'm down the the extension housing bushing.:sad: The trans was rebuilt approx 9k miles ago along with the rear diff. This last weekend I removed the extension housing and measured the bushing and didnt seem to be much different than the new one but showed wear and I changed it out. I put it all together and its still there. The vibe is kind of a come and go-you can almost time its frequency. I'm at a loss, from this point its going to start to get expensive.:banghead: any help would be much appreciated, Thanks.