Duster Driveshaft length causing Vibration?

You are not referring to the "pinion", you are referring to the "slip yoke" and I agree, it's likely out too far. a good tight one with no spline wear may be OK that way, but it seems way excessive

Also, you say the transmission centerline and rear axle pinion "are within 1/2 degree" This too may be wrong. Under power, acelleration the rear axle will "wrap up" the springs and come up. You may want it down as much as 2 more degrees