If you're lifting your truck's suspension, you need to make sure that you address every aspect of the drive train to ensure proper, consistent geometry. If you're familiar with the placement of the drive shaft and you know how to recognize it, you can measure for a new shaft easily. Here's a look at what you need to know to get the measurements.
Why Does It Matter?
The suspension of a truck is designed with precise geometry in mind because all of the components need to connect properly, and there are specific angles required for those components to move properly without binding. If you lift the suspension, it alters that geometry and can interfere with the proper function of the drive train. The angle of the drive shaft connection will be altered, and it can lead to binding, which may damage or destroy the drive shaft. To restore the proper angle, you'll have to install a longer shaft.
Measuring the Drive Shaft
In order to upgrade your drive shaft, you're going to have to take measurements to ensure that it is the proper length for your new suspension angles. That means you need to install the lift kit first, because it's the best way to evaluate the change in the drive train angles and ride height.
Once the lift is in place, put your truck on a jack and then secure it with a set of jack stands. You'll want stands on the axle shaft, directly behind each wheel. Locate the transfer case U-joint that faces the rear axle, and measure the distance between where the U-joint attaches to the transfer case and where the drive shaft attaches to the differential of the axle. Make sure you take your measurements from the flat surface of the transfer case all the way to the flat piece of the differential directly across from it. The differential, if you're not familiar, is the large round component in the center of the axle. Some people also refer to it as the "pumpkin."
The next measurement you need is the distance across the U-joint caps. Measure from the outer edge of one cap to the outer edge of the other. Note the measurement, and make sure you take measurements from both U-joints so you have measurements on the transfer case side and the axle side.
Once you have the new measurements, you can take them to a manufacturer of custom truck parts to have a shaft crafted that fits those dimensions. Suppliers like Pacesetter Truck Caps & Accessories Inc can help you craft a drive shaft that will fit properly.