But what you are describing to me sounds like a harmonic vibration in the suspension from the springs or something. Does it go fron to back or back to front. Also on the shocks on the Spring ride trucks the only place they have them is on the front axle to help keep the steering in check. On my truck it turned out to be in the spring equalizers binding up from being worn and also there was a track bar link bushing bad. That led to the same condition you have.

Yes, he is correct in the way a shock works. On a truck they are a lot stronger than a car. The springs on a truck and car are to absorb the impact. Without the shocks the truck would bounce back up to it's original position very quick. The shocks slow the process down to make it a nice smooth transition back to there original place. When you see a car going down the road and it hits a dip in the road then bounces up and down for a while, it needs shocks. Shocks do not raise or lower your truck or car if it's sitting low, that's the springs job.

If your wheels were out of balance your tires would show this very quick.

My guess it's the drive shaft.

And by the way, it's called Harmonic Frequency and everything has there limit but I doubt that it's the problem. During world war II they discovered this when army men marched across a bridge. The whole bridge started swaying back and forth and almost collapsed. Now, when the army walks across any bridge they are told to walk without rhythm.

Could be as simple as tire balance as well as blown shocks.

I once saw a trailer with both, the whole thing was bouncing down a smooth road. Wow.

Blown shocks do not cause a wheel to bounce up and down, they cause the whole car or truck to bounce up and down. If the wheels were out of balance and bouncing that much they would show signs of wear immediately, it's something else. Just ask a tire professional and they will tell you the same.

Yes the whole trailer AND wheels were bouncing up and down. That's why I thought it was wheel related. Not much else to cause a vibration like that on a trailer. The trailer wheels were coming OFF THE GROUND.

The shocks were totally gone, got them replaced and it took 50% of the up and down bounce out. I believe 25% is from the left steer tire that took a beating while the shock was blown. The tire man told me that due to the shock being so bad that the tire had to act as a tire and shock at the same time.

It did not get all of it out, but a major improvement. I am going to get the springs and bushings checked out in the future.

Thanks to everyone who helped me on this matter.