I just got a 92 bronco about a month ago, runs pretty good, but it has a weird vibration issue.. cruising along at part throttle or down a hill, no vibration whatsoever.. above about 35mph, if I give it gas and it's in a higher gear, so under load, it'll vibrate like a u joint is gonna fall out... it downshifts and goes away....

I have checked all the basics, u joints are all good, the slip yoke on the shaft has some play, but not excessive, pinion bearing has no detectable play, no detectable in the tcase... I took the rear shaft out and drove it in 4x4, and no vibration whatsoever, so that narrows it down..

the only thing that I can think of is that the rear springs are weak enough that it's allowing the axle to wrap and causing a vibration.. and they are a little saggy, best I can tell is they're the original with 152k on them.. I read somewhere that spec says the pinion shouldn't be more than 1.7° from straight with the drive shaft with a cv... and that's what mine is at right now.. but is that even a spec, or just some garbage I read on the interwebs?

I also had a buddy watch while I power braked, and he said the pinion wraped down, maybe 1/2-3/4".. would that be enough to throw it out of line?

 

I had a vibe that started above 35 MPH. Looked everything over twice.

Couldn't find the problem. Put it on jackstands and looked at the u joints on the rear driveshaft and finally saw that one u joint was bad.

Took it to a local driveline shop and they changed all the u joints and painted it for $110.00.

Those double cardan joints are a pain. I think I'll take all my u joint work to them.

 

take the rear driveshaft to a shop, have it checked for balance and possible bad joint. What happens if you grease up the slip joint and the ujoints real good out back and drive it? there should be some relief briefly.

 

I did take it to a shop, they didn't check the balance, but they checked the cv, and gave it the ok... ice had shafts out of balance before, this isn't that, as out of balance is constant, not throttle dependant in my experience

 

Little update, I got a new double cardan kit from bronco graveyard, and while I was waiting for it to show up, I flushed the transmission, it wasn't black but a not healthy brown, put a 4r100 pan on it and new filter, changed the motor oil, and went for a drive.. and no vibration whatsoever... I'm not gonna complain and save the new double cardan for when I need it