Tranny slip/ clutch chatter
If the engine and ignition checks out, is it possible the tranny is slipping? Clutch chatter? Give us a little better description. Does it act up after it shifts into OD? Is it in 3rd gear? I have 4:11's in mine and IIRC at 80 mph WOT it's still in 3rd. Let off the throttle a bit then it will hit OD. Really sounds like the engine is falling on it's face, the vibration is secondary.
Good luck,
JJ