I wouldnt adjust the ride height unless you had the exact ride height measurment availible. There are at least 3 different height settings for the rear air bag height depending on the type of air ride. Its so confusing we have a service bulletin next all the different guages that we have for measuring ride height. On the CXU and CHU you dont shorten or lenghten the rod on the valve, you actually loosen the bracket on the frame that the valve sets on and you rotate it one way or the other to adjust it. Too much up or down and it will cause vibration and even possible tear the syncro out of the transmission. Eaton Fuller likes to deny warranty for syncro damage, they claim they can tell if it was caused by improper ride height. Your new CXU should have come with a Maintance and Lube manual, in it it tells some on the ride height adjustment. I will try and find the service bulletin and post it here. Also the cab air bags have a measurment too, I will see if I can find it. We had 3 or 4 CXU's and CHU's with the vibrations when new and we installed the new motor mounts, cab mounts and rear shocks. The new rear shocks were alot stiffer than the ones that came on the truck. I always wondered if when the shocks started to wear out , would the vibration return. When the CXU and CHU first came out we still had "Mack" engineers in Allentown, I was talking to one about the vibration and he said it actually wasn't the engine vibrateing, it was a sound wave put off by the engine under a certain load, the cab was absorbing the sound wave and causing the felt vibration, kinda like when your in your car at a stop light and the guy next to you has a loud sterio with giant subwoofers cranked up and you can feel it in your car.