I noticed a lot of vibration the first time I drove the truck after installation of the suspension. It turned out the previous person who worked on the truck did not align the two u-joints phase properly. That correction took out much of the vibration. However, there was still an unacceptable amount of vibration. Here are the steps to correct the vibration problem.
Basic Driveline Set-up:
There are three pratical ways to solve dirveline vibrations. They are: u-joint with opposite angle in the top picture (most common), u-joint with same angle, middle picture, which is less common and cv-joint on the bottom picture which is quite common too.
Pros and Cons of each method:
For the Toyota set-up with Old Man Emu Leaf Springs:
Just by bolting the OME springs on and do nothing, the front u-joint angle would be 10.5 degree and the rear would be 6.9 degree. This is a problem because with torque at the wheel, the rear angle would be 6.9 degree - 3 degree(reaction due to torque load) = 3.9 degree. 10.5 degree in front and 3.9 degree is not good. That is a 6.6 degrees difference in bent of the u-joints. This is too much. Some noticable fault is when lifting the gas pedel going downhill at about 75 to 80mph there is some loud gear noise. There is gear-wining noise at all slow speeds. If this were a weekend play truck, this would be all right. As a daily driver, this noise is not acceptable. There is too much stress on the drivetrain. These gear noise and whine gets irritating in a long drive. I would not go over 65mph because of the noise.
I tried rotating the rear axel downward so that the rear and front would have the same angle. But by rotating the rear axel, the end point of the driveshaft would be lower also. This increases the front u-joint angle to 12 degree. The rear would be 12.5 degrees. 12 degrees is a little high for a u-joint angle. Although the output of the driveshaft is rotating at a constant rate, the shaft itself is speeding up and slowing down too much. This caused vibration at high speed. The axel was actually had a .5 degree downward bias from the transfer case, so when the axel wind up during acceleration, it would be aligned. This took out more vibration. However, I felt there was still some vibration in the driveline. I decided to put the heavy short leaf on the spring pack, which was taken out initially for flexibility, to make the spring the way it was designed. This final step took out almost all of the vibration. This is probably because the axel does not rotate as much with the stiffer spring and cause u-joint misalignments. However, there is still a persistent vibration that would not go away. I spent more time trying different spring and shim combination. I tried pointing the pinion flange at the transfer case with a u-joint at the transfer case. This helped some, but there was still vibration at high speed especially decelerating at high speed. My final conclusion is that I need a constant velocity joint driveshaft similar to the front driveshaft. Using Auto-Cadd, I found that rotating the axel 4 degrees up would give a 2.8-degree bias on the back u-joint. With acceleration, that 2.8 degree angle should drop down to 0 hopefully because of torque on the axel housing during acceleration. I used Auto-Cadd to figure the geometry out because I am tired of the trial and error method. I got a brand new cv shaft from Inland Empire Driveline Services. They bought a large batch of Toyota cv shaft long time ago, and still had a couple left. After paying $480, I installed the new shaft and a 4-degree shim. That took out all the vibrations
In the decision on CV driveshaft, there are two options. Use a cv-joint on one end of the driveshaft which is common practice, or to use 2 cv-joints at both ends of the driveshaft. If I use one cv-joint, I would have to rotate the axel up so that it point at the transfer case. This would require a 3 or 4 degree shim. This slight amount of rotation should not cause any oil starvation problems. The other option is to use two cv-joints one at each end of the driveshaft. This will give a smooth ride no matter what angles each joint is at. I chose the one cv-joint for cost savings. After the installation of the CV joint, the ride was as smooth as it was before the lift. In all, I had to install and remove the axel many times trying different solution and taking out leaves to come to an acceptable solution.
The conclusion is that even with a 2.5" lift, a CV driveshaft is absolutely necessary if you want the truck to be durable. Without the cv shaft, the u-joint angles are running too high causing shutters and gear noise. For the most part, since the vibration is torsional, and the engine and transmission is mounted on rubber mounts, you will not feel anything other than a slight annoyance. It is easy to ignore that slight vibration. However, that shutter is doing harm to the gears and bearings throughout the driveline. If you intend to sell the vehicle after a year or two, or if the vehicle is only used on the weekend, then there is no point spending the extra money on a cv shaft. However, if the vehicle is a daily driver, then a cv shaft is a must. That slight vibration becomes a major annoyance on long trips especially when you know you are destroying the vehicle as you are driving it. I hate to be faced with a large repair bill time and time again because the drivetrain was not set up correctly in the first place. With the CV joint and the 4-degree rotation of the axel, the ride is absolutely smooth. That is the way that a car should drive in the first place.
Lots of technical information on u-joint and CV-joint: