Agree with skidnark. The driveshaft with a GV is very short and working angles of the u joints is critical. I run stiff springs with a pinion snubber 1/4" from the floor at the track and pull the pin to drop the snubber on the street. I chased a vibration after the GV install and when the shop shortened the shaft the u joint phasing was off by 3*. I was pissed and chased pinion angle. My trans is 1.5 down and pinion is .5 down. Checking the u joint phasing first.