5th and 6th vibe?

[2006is350]

I am getting a vibe going into 5th and 6th at high speed in the shifter. I can feel it as i push it into each gear, only 5 and 6. Any idea what it could be?

 

[Jomo1994z71]

Does the shifter knob shake back and forth? Mine vibes a little but I guess it's normal:shrug:

 

[Avenger]

Pinion angle / excessive driveshaft runout / imbalance

http://www.svtperformance.com/forums/showthread.php?t=407597