I borrowed this truck over a year ago and it was shaking then. It was assigned to me several months ago and it still shook. It's been through several sets of steers unless they keep lying to me and didn't really change them. The shaking never changed.
I would have thought at some point it would have stopped shaking if it was a tire balance issue. They possibly couldn't all be bad.
Like I said it was shaking even when the alignment was good.
You might be onto something with the bearings but I don't recall in cars ever having a bad bearing that smoothed out more at highway speed. It was always worse the faster you went.
Whenever we have steering shake, we always go to the steer axle without consideration to the rest. Make sure you have ALL the wheel bearings checked. I had a similar shake on my ProStar and it ended up being caused by the drive wheel bearings. All of them were shot and I had to get a spindle replaced. Also, check the torsion bars back there.
Most large carriers don't bother balancing tires. They just slap them on and send you down the road. This could be a source also.
If the tires are mounted on the rims correctly, there's not a lot of slop between the rlm and hub or studs - balancing really shouldn't be part of the problem. However on fleet trucks that can certainly add a lot to it. Of course, being a mega and a fleet truck, its doubtful those tires are ever going to be balanced.