Peterbilt Viration Problem

I have a 2005 Pete 379 EXHD day cab. C-15 with a 13speed. Currently I am experiencing a vibration at highway speed (60-70Mph) from somewhere. It has good drive and steer tires bought 7000 Miles ago. They have been checked buy the business where they were bought for out of round, out of balance, mismount, defects, bent wheels and have been road force tested. Still no results. Bobtail it isn't bad, mostly happens at 65Mph and isn't that consistent. I'm not sure if it is from the road and my shocks are bad and the trucks continues to bounce after each bump a bit or it is in the truck somewhere. With my trailer (tripple axle hopper trailer) it is at its worst empty and not there under load. The U-Joints seem tight and all are phased, new hanger bearing, the shocks were replaced when I bought the truck 15000 miles ago. I am not sure where to start, if it is my trailer, truck and my trailer is amplifying the problem or both. I have been having this problem for awhile now and have not been able to get to the bottom of it. So any help is greatly appriciated. Thanks in advance!

 

The tires seem to be wearing real well, no uneven tread wear or cupping. I have checked wheel bearing play by jacking the truck up and trying to wiggle the wheels with a large bar and nothing I can see. Is there a better way to check for wheel bearing play? I have not checked either yokes yet. I was thinking that low frequency vibrations indicate tire, suspension those types of problems. It isn't a very fast vibration, its a slower hopping like a gallop. Having said that I am going to run out to the shop and check the yokes.

 

No play in any yoke, slip yoke, or U-Joint

 

Also it has hub piloted aluminum wheels, and I installed centering nuts. Which didn't help either.

 

Revmup That sounds like what my problem is. My neighbor is a commercial trucker and has a few trailers. When he has one empty I am going to give it a pull to see if it behaves the same or if the shake goes away. I suspect the trailer as for when I drive bobtail the shake almost entirely disappears. Did you have to put new tires on your trailer?

 

bbt_387 I have heard of those slick disks, might be a good try. I'm not sure but I don't think they would dampen the vibration very much. Unless there is another reason I should be using one?

 

The vibration feels like its behind me, I feel it in the seat.