Bad vibration under load

[sag2]

I changed the ujoints in the rear driveshaft with the greasable ones from Napa.



I started thinking that it could be driveline angles especially since I have airbags. I noticed that it would vibrate bad when I had a trailer on it and airbags aired up to 70 psi, but wouldn't vibrate when I was not pulling a trailer and the air bags were empty. I loaded up the trailer and didn't air up the bags and noticed the vibration was hardly noticeable.



I have also started noticing that it is vibrating when I first start out (like the launch shudder on steroids). I am going to have the driveshafts balanced and see if that helps.

Don't bother with the balance, it will not change a thing. You have a second order vibration (angle). I'm a little confused that it does not do it with no air in the bags, but does it with the bags inflated. Loaded that is usually backward of what you should see. I think it was asked already, what is the unloaded vs. loaded rear ride height with the bags empty and inflated? This should be a simple drive line angle problem, as you have proved with the bags.