Help Troubleshooting Driveline issue

[RJB44]

I have a 1991 GMC Jimmy, which has a 4 inch lift (springs). I bought the truck with the lift already installed (installed 2006). Truck runs 35.12.50 on 15 X 10 wheels. i have a hard vibration on acceleration from a stop or when acceleration from low speed. The vibration seems to level out after 30 MPH or so. Most noticeable at 10-25 Mph. I have replaced u-joints and balanced and rotated tires. Vibration is still there. I know the transfer case was lowered about one inch previously. I don't believe the drive shaft was altered. Some people have told me to remove the front drive shift, change hubs from automatic to manual, and have drive shaft lengthened and balanced. I'm in the process of elimination now, but need to know if I should bother with removing front shaft or hubs, and just go and have the driveshaft lengthened. A couple of people have said that is most likely the problem.

If so, can I just remove the rear shaft and bring it to a shop, or do I need to bring the truck itself. Is there a standard length for 4 inch lifts?

Thanks

 

[4X4HIGH]

Here is the problem with lifted short wheel based vehicles. When you lift a vehicle the driveline angle changes. The output shaft of the t-case and the pinion need to be parallel with each other and when lifting this changes. Also the working angle on a u-joint should not exceed 3* which when lifted does exceed this limit. basically, you can correct the parallelism but not the working angle, this is why a rear CV driveshaft will cure the problem. Yes they aren't cheap but it's better than dealing with a vibration that never goes away and keeps eating u-joints. When you run a CV rear shaft the pinion needs to point directly at the center of the driveshaft (actually 2* down as this accounts for axle wrap while driving).

 

[throttleissues]

Wow, 3* is awfully stingy on the working angle, but otherwise x2 on what 4X4HIGH said.

 

[4X4HIGH]

For those that don't know what causes the vibration i'll tell you. A u-joint travels through an arch as it rotates, the front and rear u-joint need to travel at the same speed to cancel each other out to eleminate any vibrations. If you have a different working angle on the front u-joint than you do on the rear u-joint this means one u-joint is traveling faster through the arch than the other and now you'l have a vibration. I know it's hard to believe that 2 u-joints on the same driveshaft can actually move at different speeds. In actuality they are moving the same speed but not through the arch. There are videos out there that will help you understand the process and how it works. I've watched one of these videos years ago and it made me understand the what and why of it.
http://www.4x4wire.com/toyota/tech/driveline/
Here is some really good reading that explains it all. http://www.4x4wire.com/toyota/tech/driveline/

 

[throttleissues]

Essentially the shaft speeds up and slows down, but if the u-joints are in phase and at equal angles, the input and output spin at a constant speed. With a CV, you basically make all of this happen in the 2 joints of the CV and then set the 3rd joint nearly straight to keep all spinning at a constant speed. Not trying to step on toes, just trying to explain it a little simpler.