Hey gang, My truck is all but undriveable due to a really bad vibration in the driveline. It is an all-stock '89 Xtra cab ADD 3VZE with 263,000 miles on it. The vibration occurs above 35 mph and gets exponentially worse with speed, but goes away immediately when coasting or with the clutch depressed. I feel no difference with engine speed, only vehicle speed, and it's the same in 2Hi or 4Hi. I crawled under my truck and found that the center bearing was shot, so I replaced it with a new one, but the vibration remains. All of the rest of the driveline feels good with no signs of looseness at any of the U-Joints, T-case flange or diff flange and everything was recently greased. The vibration comes through the seat, steering wheel, and gas pedal and is bad enough to send the body into a resonance. It feels like I'm driving on a washboard dirt road. I can accelerate moderately from a stop up to around 35mph with no vibration issues and I can free-rev the engine smoothly.

Since it smooths out with the clutch in, I have ruled out tires. I haven't ruled out engine or trans mounts since they wouldn't be loaded during coast, but I'm not sure that a bad mount would cause this severe of a vibration and I don't see anything physically wrong. Also, if a mount was the cause, it should vary with engine speed and load rather than vehicle speed. The driveshafts are all straight with no signs of damage.

I still need to remove the rear driveshaft and roadtest it in 4Hi, but the weather sucks and we have lots of snow and ice on the roads.

What am I missing? I searched the archives, but the only things I found were center bearing, and lifted trucks with angle issues. Could it be the rear axle? I don't hear any grinding or clunking.

My summer car is hibernating, so this is all I have to drive for the next two months. I keep it below 45mph by taking all backroads to work, but even at those speeds, the vibrations have to be hard on the truck. I need to find the root cause soon. <img src="/forums/images/graemlins/confused.gif" alt="" />

Might have a look at this:
- http://www.4crawler.com/4x4/ForSale/Shims.shtml#FAQ1

I'd yank the drive shaft and have a closer look at everything from the rear output on the t-case to the input on the diff, and both the rear ujoints. It has to be one of these.

I saw that while I was doing research, but dismissed it since this vibration came on rather quickly without any changes to the truck. Had I lifted it, or had previously disassembled something, I could see either driveline angles or phasing being possible causes, but it has been fine for 20+ years and nothing has been apart. I did make sure I marked everything before taking the driveshaft apart, so the phasing didn't change.

While apart, I pushed and pulled on each joint and flange. They all felt perfectly tight with no runout. The transfer case and differential did, however, have a lot of lash, but no worse than any other time I've tested them.

When I replaced the center bearing, I had the forward rear driveshaft completely out and both U-joints felt just like new. I did not disassemble the U-joint at the diff. I only shook it while attached.

I was 100% confident that replacing the badly worn center bearing would be the final cure, but somehow, as bad as it was, it wasn't the cause of the vibration.

I've had U-joints go bad before on other vehicles, but they have always felt wrong when checked by hand. They either had movement within the joint, or they were overly floppy with the flange unbolted.

Might still want to measure the shaft and flange angle. With more vibes under power and it goes away while coasting that may be telling you that the pinion flange is angled up too high. Why is this? Pinion will tip up (higher) under load due to the springs twisting upward and when coasting the pinion will drop back to it's rest angle. Might be it is right on the "hairy" edge, fine while coasting but with a few degrees of up-tilt under load, it may vibrate.

Gotcha. I will measure the angles and see if I'm at the edge.

What's weird is that I can be going uphill or downhill and the second I apply any throttle, it vibrates. The loads have to be tiny when just trying to maintain speed down a small hill. Keep in mind that Michigan is very flat, so it's hard to find anything with any real grade.

Thanks for the help.

You'd be surprised about mounts and u's. They can fool you.
Father-in-law's Chevy pu 350/350 started vibrating ONLY! when he was in a rt hand curve. NAH!! never heard of that.

Drove it. It did it. HUH!!

New trans mount fixed it. Softened on one side only and would drive you nuts. Check 'em out.

This is kinda long shot, but related to 4Crawler's point - I wonder if you are having axle wrap problems. Could'has anything torqued your rear end lateley?
e.g. maybe after so many years, a spring is simply failing/warping, but only under load. That woudl cause the pinion angle to be off, but only when accelerating.
Heck could even be a loose bolt somewere in the suspension?

here's an idea (nice and dangerous too!)
Jack it up so the rear is in the air and very well supported. Have a buddy watch the driveline while you apply the gas, watch the speedo and see if you still get the vibes like that. They maye be able to see how much movement there is.
9when I did this recently I tied the rear bumper to a tree for safety ;-) )

I would for the heck of it replace the trans mount first, then the motor mounts. Mine looked and felt fine but that was the fix.

Also check for play in the cab mounts.