05 379X bad bounce down going down the road

Your description is bang-on for something out of round with one of the drives. When I encounter a complaint like this I put both drive axles up on jack stands and run the truck to speed in the shop. You will find one or more tires bouncing. I know you said the tires have been checked for roundness, but it takes so little out of round to cause a big shake! You really need to see how the tires are rolling with those axles jacked up. Tires can be mis-mounted on the rim, so the whole wheel is spinning out of round. Rims can be slightly off center on the hub, and this also causes the wheel to spin out of round. Centramatics will do nothing for you, they are only for fine tuning the balance of a wheel that is running true. The tread wear pattern may simply be the wrong tread design for your application and rarely to blame for the type of shake you are describing.

 

How did you turn them, by hand?

 

When you spin the tires, also check for a bent rim. Alum rims bend just like steel, happened on one of my steers.

 

I had a cracked pressure plate did that once. It was funny to find it out even but some oldtimer knew what it was.

 

Nope, complete waste of time. You'll need to run the truck to 30 mph or so and watch the tires. Trust me, you will be amazed by how much radial out of round you will see! I pick out the stones, for safety. Most trucks have traction control, so I pull the ABS fuses. I just put the truck into top gear at and run it at idle. It is usually around 25 or so mph. With the wheels spinning you can really see how much radial or even side to side movement there is. There is always one tire that sticks out. Once you see it you will know what I mean!

 

Sounds like C4½ may be required.

Stationary & spinning, don't overlook the driveshaft or any other rotational items. Also, have you made sure your bags have proper inflation and height valve is functioning as it should? Just slingin' out some thoughts.