Axle bounce

I have 2 386 Peterbilts that both have developed a sort of hop at speed. I notice it when I drove one the other day. At first I thought it was the spring ride trailer I was pulling. After a while I realized it was the truck and it only seems to do it when I'm going about 60 to 65 and when there is little to no tension on the driveline. If it starts bouncing and I let out of the throttle completely or hammer down, it stops. It feels almost like a hub pilot is worn but it doesn't do it all the time. I said something to the driver of my other 386 and he said his does the same thing. They both have Flex Air suspensions. Motor mounts? Suspension bushings?

 

It's greased up and slick as can be.

 

I forgot to mention that in a tight turn (like turning around in a parking lot or leaving a truckstop) you can feel a shudder as well.

 

No, the vibration is too slow to be the driveshaft. It's more of a bounce, as if driving on egg shaped wheels all of a sudden.

 

Yes sir.