KW vibration

Hello! First post...

I've a KW T660 with a vibration issue. Noticeable above 60 mph. The thing that has me a little puzzled is it only happens when coasting or maintaining highway speed; any load or stress on the drivetrain and it smooths out. (I.e. acceleration, braking, engine braking). Locking the power divider makes it a little less noticeable.

The yoke going into the power divider has a little play, so I know it needs a little attention there. The carrier bearing also has about 1/4 inch of play, but that is pushing up pretty hard on the driveshaft. I can also turn the driveshaft several degrees... The company truck I drive sometimes has the same slop but no vibration. U-joints seem OK, no play in any of them.

Any thoughts? I haven't found much on here so if it's here redirect me. It worries me because I had the same vibration in another truck right before the power divider started breaking teeth and shelling the front carrier...

This is a higher frequency vibration so I don't think it's a tire or brake issue. Any thoughts? Motor mounts, maybe?

 

A couple of guys are coming into the shop this weekend to have a look, going to spin it up with and without wheels. I'm going to put new shocks on the drive axles while it's under shelter, since 3 are bad. I have messed with the ride height; spec is 8.5" and I have gone on both sides of that with no change. It is set almost spot on now.

Now that I've been in it for a couple of weeks I'm still a little puzzled. Empty it is very noticeable, but loaded and it almost goes away completely....BUT It comes and goes. Occasionally around 60mph loaded it gets REALLY bad. Sometimes it rattles your eyes out at 55, sometimes its smooth as glass all the way to and from 80. I'm still believing this is a driveline issue over a tire issue due to the frequency (think rumble strip at 20-25 mph)

U joints are OK, lash is OK, slip yoke and slip joints are OK, as checked by a few of the mechanics. Carrier bearing checks out too, but I may replace it soon anyways.

It might be worth mentioning that it's the AG380, and truck has 500k on it. Dealer also threw some cheap drive tires on it (Leao?) And spray painted everything underneath black, so it's hard to tell what is worn out. Short of replacing everything behind the clutch I'm about out of ideas. Hopefully this weekend I'll know something.

 

I wouldn't have thought to look at the trailer, though the one I pull could use a little love mechanically. However my vibration is present with or without the trailer. Much worse when bobtailing. The trailer I pull also caused me no issues for 70k miles behind a company truck, aside from the brakes being a little snug.