Axle hop

I have a W900L with the AG380 (Flex Air) suspension. The truck has had a bounce to it since I bought it last year. Nothing I do seems to help it. The bushings look ok, I have replaced all of the wheel bearings, brakes, both differentials, all new steel wheels (outers are aluminum), all the u-joints, all the shocks, sticker tires all the way around and had them all balanced. I also checked the axles to make sure they are straight. I recently slid my 5th wheel back to center the kingpin between the axles and the bounce seems to have gotten worse. When I'm bobtailing the bounce comes in at 30, goes away, then comes back between 55 and 65. When loaded, it comes in about 60 and only gets worse the faster I go. Any ideas? I've about had it trying to chase out this gremlin.

 

By bounce I mean that it feels like my tires are going down the road like basketballs go down the court. It isn't a vibration, it really feels like the back axles are trying to come off the ground. Like I said in the original post, when I'm bobtail it comes in at certain speeds, when I'm loaded it just gets worse. I'm out of ideas.

 

I'm not sure what you mean by "steady bearing." I'm assuming you are talking about the carrier bearing, in which case yes, it was replaced recently and I have inspected it since. The yokes are all good as well, all of the nuts tight as can be.

 

I was wondering if the intermediate shaft might be bad, but the mechanic says it doesn't have any play in it.

 

Yeah, we checked them for run out and they checked out ok. We did find 1 steel wheels was a little bent, so I replaced them all. Nothing. I forgot to mention I also made sure the ride height is set perfectly per kenworths specs.