Drive shaft un balenced?

[svxsubaru1]

I converted my car to a 5spd, and used the small car basic install kit, so iam using the drive shaft extension. When ever i go above 70 MPh there is a pretty bad drive train vibration, i think my engine taps the hood . what do you think it is, is it a unbalenced drive shaft, becasue of that stupid spacer, or is it something else. Isent the 2 gen legacy 5spd drive shaft spose to be the righht lenght? Thanks