Axle hop

Maybe a shot in the dark but how about someone ride beside you in a car for observation?

 

You need an inclinometer and Kenworth's specs for the driveline angles.

 

Check the drive shaft lengths. If they are too long the slip yokes will bottom out causing a symptom like you describe.

TA

 

Did anyone do a on axle balance with the tires?

 

You need to set up both drive axles on jack stands and run the truck and watch the tires. I think you have one or more that is running out of round. It may be very well in balance, but the wheel probably isn't centered properly on the hub. Running the truck when up on jack stands allows you to watch the wheels as they rotate. If they are mounted off center they will go up and down as they are rotating. If you run them too fast they are just a blur and hard to see/spot. I always have a second person in the cab when I do this and run the truck in a high gear, with the engine at idle.

 

Exactly what heavy said.

You got a wheel group not centered on the hub properly. Only way to get a hop that bad , you have to have something really heavy not centered.
Drive shafts are vibration related .
A bounce or gallop or hop is almost always a wheel group outta wack somehow.
I've seen aluminum hubpilot hubs with so much corrosion the pilot is gone.

The first thing I would of done it put truck up on jack stands and spun it up.

 

After more thinking a set of true balance centering pins might do you wonders .