2nd Gen Non-Engine/Transmission Rocking back and forth oscillation

[Iron Duke II]

My 96 4x4 started rocking back and forth (Vehicle oscillates) while between 30 - 45 mph with light throttle or coasting a few weeks after having the u-joints replaced from the transfer case to the rear diff, carrier bearing, had steering box rebuilt and a T-style steering kit from Synergy installed w/new steering damper. The shop did a front-end alignment and replaced one of the control arms that had badly worn bushings.

The oscillation could be due to several factors, and here are a few possibilities I am reviewing based on similar experiences shared:

Driveshaft Balancing: U-joints were replaced and the driveshaft may need to be properly balanced, it could lead to vibrations. It may be the driveshaft was reinstalled without precise alignment to its original position. Even a slight imbalance can cause noticeable oscillation.

Driveshaft Indexing: Improper indexing of the driveshaft during reinstallation can lead to vibrations. It’s recommended to try reindexing the shaft or, if necessary, redo the u-joint installation.

Missing Counterweights: Small balancing weights on the driveshaft are critical for maintaining balance. If one was knocked off during the repair, it could be the source of the problem.

Control Arm and Steering Components: Since a control arm was replaced and a new steering kit installed, ensure all related components are tightened to the correct specifications. Any looseness could contribute to instability.

Alignment and Suspension Check: A comprehensive check of the alignment and suspension system may reveal issues that weren’t apparent during the initial alignment.

Given the complexity of diagnosing such issues remotely, I am returning to the repair shop to have them take a second look. They should be able to diagnose the issue, especially since it occurred after recent work was done. Any suggestions? Thanks

 

[petersonj]

Can you provide more detail to help us understand vehicle "rocking back and forth" (oscillation).

The closest thing for me that could match your description could be a rear driveshaft that was one spline tooth (or more) out of phase. I don't know if installing a drive shaft out of phase is possible on your truck. Some driveshaft splines are designed to be installed in only two positions, but both positions would still be in phase.

The photo below shows a spline connection point. Note that the end yokes on each joined piece are in the same plane (in phase).

- John

 

[Iron Duke II]

The sensation feels like the whole vehicle is shaking back and forth, forward and aft in the 30 -45 mph range. Not sure how else to describe it. I'm taking it in in the AM tomorrow for tire balancing and rotation, before bringing to the shop that did the work described.

However, It may be the shop misaligned the phasing when replacing the u-joints.

 

[Ozymandias]

Forget the driveshaft, this makes Vibrations but not rocking motions.
And Vibrations would be at a higher speed, not rolling around with just 30mph.
Also if it would be the driveshaft the problem would have been there instant after the job was done and not weeks later.

Fore and back motion I only know from bad aka. worn injection pump where it can't keep the rpm steady even with a steady throttle. Especially a light load less then 20% demand or so.

You probably have no rocking motion with higher load right?

 

[Iron Duke II]

Forget the driveshaft, this makes Vibrations but not rocking motions.
And Vibrations would be at a higher speed, not rolling around with just 30mph.
Also if it would be the driveshaft the problem would have been there instant after the job was done and not weeks later.

Fore and back motion I only know from bad aka. worn injection pump where it can't keep the rpm steady even with a steady throttle. Especially a light load less then 20% demand or so.

You probably have no rocking motion with higher load right?

 

[Iron Duke II]

No rocking motion with higher load noted so far. It would be disappointing for a newish pump from Scheid Diesel, sourced to my mechanic from Austin Injection in 2019, now with about 14K mi to already have an issue. The fuel filter is changed every year. Maybe I should have the fuel pressure checked, although it has a new Cummins lift pump too. It starts up great, idles and accelerates perfect. Again this issue started after the driveshaft u-joints replaced. I did fail to mention the u-joints on the front end were replaced and a new carrier bearing was replaced.

 

[BigPapa]

No offense to Ozy, but I doubt it's any kind of fuel issue. The 12-valves either run or they don't. I'm like you, I'd go after the drive line first.