This may be a stupid question, but my 2001 2500HD Auto 4WD 6.0 xc long bed has a bad shimmy in it between 40 and 55 mph. The truck is completely stock and I just had the tires rotated and balanced around 100 miles ago. Which by the way helped, but very little. I was thinking it was the driveshaft needing balanced, but the truck only has 70,000 mostly hwy miles and no off roading at all. Any help would be great. Thanks in advance.

Sean

 

Could Be A Tire Seperating. How Old Are They?

 

At worse it would be your tranny acting up. Maybe the toque converter is having problems locking up into OD.
Have you ever serviced your tranny? At least a pan drop and filter change. Recommended every 25,000 mi. The owner manual is wrong.
Ask your local dealer service rep or most tranny shops.
The other problem would be a bad tire (separation/ bent rim/ bad tire wear).
You did say the vibrations did settle down a little after the balance and rotation. Maybe the bad tire or tire(s) are now on the rear. You will feel less of the vibration with a bad tire on the rear.
I would go back and have your local tire shop really inspect your tires for uneven wear or a separation first. When the tire tech spins your tire and wheel on the balancer, watch the tire as it spins on the balancer. I always look at my tires when they balance them.
Oh yeah, get an alignment too. Many people neglect that portion of maintenance. Just because your vehicle doesn't pull left or right means NOTHING. A pull will have to do with either a tire causing a "radial pull" or bad caster settings. Alignment settings that wear your tires will have bad camber and toe settings. Though neither bad toe or camber settings will cause a pull.
Also if your Caster adjustments are way off it will cause you to loose control of your vehicle at high speeds.
One more thing to check- U-JOINTS. Make sure your or a mechanic inspects them closely. A bad U-joint will also cause a vibration.

Jim

 

Thanks for the info. I have very closely inspected the tires and have not found any noticable problems. Though it probably would not hurt to replace them, I think they have about 4o K miles on them. I will say the shimmy goes away above 60 mph and the truck runs smooth at hwy speeds. This is why I was thinking the driveshaft might need balancing. Thanks for the ideas, I now have somewhere to start.