Determining source of driveline vibration (1 Viewer)

[earl]

I'm trying to determine the source of a driveline vibration.

Details:
- BJ70, with OME 2.5" lift (rear sits 1 inch higher than front)
- 80 kph and over in 4th gear when I let off the accelerator I get a hard and fast vibration (about 2500+ rpm).
- Same speed in 5th gear when I let off the accelerator I get little to no vibration.
- Under acceleration I get some vibration but it isn't as bad or same as under decel.
- Coasting in neutral at same speed there's little to no vibration.
- When I drive without rear driveshaft the vibration under deceleration is gone, but I do get some weird noise from my tranny/t-case... like something is loose inside. Not pleasant sound. Not sure if it's normal due to driving with only front driveshaft? Seems I remember the same noise from another vehicle I drove without a rear driveshaft...???
- Rear driveshaft has no looseness, play or rough feeling in u-joints/splints. Driveshaft yokes are aligned properly.
- Rear end has no play or rough feeling when turning by hand
- T-case has no play or rough feeling
- Tranny/rear mount appears to be in good condition and the rubber is still bonded to the metal.
- Exhaust is new and isn't banging on anything.
- PO "replaced all diff side bearings and pinion bearings".

At this point I'm wondering, is it possible for a rear differential to vibrate? I've had a couple people tell me no.

Does this sound like engine mounts for those of you who have experienced actual bad engine mounts?

 

[ForealBoreal]

somethings out of balance.
has this happened since day 1 of owning the truck?
when did you change the u joints?

 

[cicak]

long and short shafts are not bent?

u joints have all been changed?

busted or off Engine mounts you would be able to see stationary with the bonnet up every time you rev the engine it will twist.

might as well check the gearbox mount also.

 

[earl]

The u-joints aren't new, but "feel" good (i.e. moves freely, and tight). I also took the rear shaft to a driveline shop today and they also said it seems fine. They said they could balance it if I wanted for $70.

I was thinking about picking up a protractor or angle indicator/finder and check the driveline angles. It looks okay visually, but maybe it's out, or maybe under load it changes enough to cause a vibration.

Does anyone else with a BJ70 and OME lift *not* have vibration issues on the highway? Are you running shims in the back?

Thanks

 

[earl]

I had someone let off the clutch in forward and reverse while I watched the mounts. None of them appeared to be separated. They did flex.

Going to rule out driveline angles before I do anything more.

 

[roscoFJ73]

Does anyone else with a BJ70 and OME lift *not* have vibration issues on the highway? Are you running shims in the back?

Thanks

Generally ,a 2 inch lift will not cause problems,but it can make an old u joint noisier.

 

[earl]

I checked the pinion/flange angles, and they are equal and opposite. I'm not sure what is an acceptable driveshaft angle though?

Amazingly, I was able to source some engine and tranny mounts through my local auto parts store. They knocked a big chunk of $$$ off the list price so they're very cheap. Not exactly sure what they'll look like, or who makes them, but I should see them next week.

 

[earl]

Well my amazingly cheap aftermarket engine and tranny mounts came today. Unfortunately, but not surprising, they were totally wrong and looked like they were for a dinky car given their tiny size.

 

[earl]

Well, found out today that my rear driveshaft slip yoke is worn out with noticeable play. It's a little strange that neither me or the driveline shop didn't feel it before. The only thing I did since was pull it apart, and squeeze out any excess grease.

Took it for a drive with only the front driveshaft again and WOW I can't believe how smooth it is. I can accelerate to 3000 rpm, completely smooth! Feels like a new vehicle.

I have a feeling the driveshaft is just the start, and that I'll be needing a new rear end also. Guess just because it feels tight, doesn't mean it is.