Just bought this truck. Has 150k miles. 5 speed auto allison tranny. Every single time the trans hits 5th gear, it vibrates. Like out of balance driveline annoying drone vibration. If its in 5th gear at highway speed, its vibrating. All the way passed 65-70 mph. And the driveshaft, carrier, and u joints are all new. Trans shifts excellent, never slips, just vibs in 5th. Please help.

 

Have you balanced your tires may have a bad tire just a suggestion

 

Is it just fifth gear or is it more the speed that makes it do it?

 

its JUST 5th