"Galloping," or speed related "hopping".

It is common to have wheels that do not self center on hubs or hubs that are not machined to center a wheel on it, and therefore the wheel itself is running out of round causing your gallop. Best way to find it is putting both drives up on jack stands and running the truck at about 30 mph or so and watch for the tire that is going up and down as it is spinning. This is sorta tricky, if you spin them too fast, they are just a blur and will look perfect, but too slow and you can't pick up the osculation either. We use extreme caution with this test, always have a second person in the cab with the steers tires fully blocked and all stones picked out and keep away from spinning parts. Be safe if you perform this test.

 

Are the driveshafts phased properly? The yokes on either end of each shaft should be aligned to each other.

I have had a bad u-joint feel like a hop, almost like someone was kicking the bottom of the seat at the exact same speed every time. That was on a half ton truck mind you but I'm sure the principal would apply to a large truck as well.

 

That sounds like a horribly dangerous test Heavyd.... Maybe he could just block the wheels, jack the truck up, release the maxi brakes, put a jack stand next to the wheel about a 1/2“ or so away from the tread and the spin it by hand, that should be plenty to tell him if it's out of round. Just a thought!!!

 

Yes, it is very dangerous and that is why it needs to be done properly and safely. But you would be shocked at how little a tires need to be running out of round to cause a major shake. You'll never see it turning it by hand.

 

I think you would definitely see it if you put a jack stand next to the tread so that you have a reference point. Probably not if you're just looking at it without anything there

 

I have found problems like that just as HD. Did not run them quite that fast. Tightening the steer axle brakes all the way down helps with stability. Lock the P/D.
Reminds me of two cases. First had out of ballance drums. They had no external ballance weights to stop them when rolled on the floor. Rolled a few rotations, stoped and came back, then reversed again. Second was a off center hub.

 

Easy rule of thumb to check ride height on KW Ag100 air ride. Slide a 2x4 between the axle stop and the top of the axle. It should just fit.

 

Check to see if you have the right brake drums installed, if the drums are not piloting up correctly they will throw out the wheel balance.

I had this happen once on one axle, I had the brakes replaced and both drums but they installed the wrong drums and it hopped at 30 and 60 so bad I was sure it would break something if I put a load on it.

 

30 mph on jack stands?

Please let us know where your workshop is.... So we never come near it...

That's dangerous all hell...