Front Drivetrain Vibration

I have a 2003 Sahara with a front drivetrain vibration, at speeds over 50 MPH, that I have not been able to resolve. I have scoured the forums and followed many suggestions, but still have not been able to resolve...hence a separate post to see if there are any other ideas. Basically, the vibration is only present when front driveshaft is in, and I recently discovered it is not felt when in 4WD under load (vibrates in 2WD or 4WD if coasting). Running on 31s, no regear or change from OEM differential.

Here are the details of the background and what I have tried so far:

I installed a 2" lift w/ a SYE (to eliminate rear drive line vibrations) two years ago, and all was well until I had to rebuild my front OEM driveshaft (centering joint was chirping). Shortly thereafter, the vibrations started. So...
- Removed the front driveshaft (and have done multiple times since). No vibrations up to 75+mph.
- Took OEM driveshaft to a shop to have it inspected & balanced. Vibrations became worse (starting ~40mph).
- Reading some forum posts about not being able to get an old OEM driveshaft to balance right, so installed a new Tom Wood's driveshaft. Vibrations became better, now occurring above 50mph. Read Tom Wood's post on possible vibration sources.
- Installed adjustable control arm cam bolts to adjust the pinon angle. Pinon at 6, driveshaft 6.5 (so 0.5 delta). Vibrations got slightly better. Very slight at 50mph, but uncomfortable 55+.
- Checked yokes. Both seem solid. Some slight in/out play on transfer case side. Took it into a shop for an inspection to see what I was missing. They could see no issues, except for concern with the axle u-joints.
- Replaced both axle u-joints. One was bad. No change to vibrations.
- As noted above, I just discovered that there is no vibration in 4WD if under load (accelerating or maintaining speed). Vibration immediately comes back if I stop accelerating & coast, but then goes away upon acceleration. I have no idea what this would indicate.

Any other ideas?

Thanks in advance.

JMT said:

What about this? I assume you are talking about on the Tcase out to the front driveshaft.

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Yes, the output yoke from the Tcase to the front driveshaft. There is slight movement (maybe 1/8") of movement axially, in & out of tcase. No side to side movement & nut is tight. Transmission shop said it was a normal amount of movement, but it was a concern I asked them to check.

Blackjack said:

Try increasing your caster by half a degree and see if that makes a difference.

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So that the delta between the pinon & DS is closer to 1? Also, does it matter if the front pinon is pointed below the driveshaft (and therefore more caster), or above (less caster)? Currently pointed below (6 vs 6.5 for DS).

Blackjack said:

Try increasing your caster by half a degree and see if that makes a difference.

Click to expand...

I adjusted to ~1 degree, trying both higher & lower than driveshaft w/ no difference. Figured while I was at it, would try some other also, going up to ~1.5 w/ no difference. In all angles, vibration still goes away in 4WD while maintaining speed (but come back if coasting).

I know I said the shop didn't think the play in the yoke on the transfer case was an issue. Does anyone know if there should be any movement of the yoke on a NP231 transfer case?