HOW ABOUT THE REAR TRANSMISSION MOUNT?....

 

Have you checked the grease on the slip joint? That's the only other thing I can think of right now. My center carrier has play in that bushing, and that definitely causes me some vibe around 40 mph. Just depends on how much torque is put on the whole driveline; in gear there is some torque being applied, but out of gear there is almost reverse torque being applied as the rearend spins the 'shaft.

I'd probably start with that center carrier (that's what I'm going to do when I've got some time), and while it's out make sure the splines have enough grease. Past that... well, those other things probably needed attention anyway!

Clay

 

Keep me posted i have a 6.0 with 6 speed trans. Doing the exact same thing except the truck shakes violently. I replaced carrier bearing due to a flat bearing. Didnt help, but the bearing needed to be replaced anyway. My u joints feel good but i am going to replace them just to rule them out and then have shafts balanced.


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How would I go about checking the rear transmission mount?

How do you know if the grease on the slip joint is bad? Is there a grease zerk so you can grease it?

 

How would I go about checking the rear transmission mount?

How do you know if the grease on the slip joint is bad? Is there a grease zerk so you can grease it?

ON MOST TRANSMISSIONS.....YOU CAN VISUALLY LOOK AT THE MOUNT TO SEE IF IT IS CRACKED OR BROKEN.....

YOU CAN TAKE A JACK AND A BLOCK OF WOOD AND LIGHTLY PUT PRESSURE ON THE END OF THE TRANSMISSION AND SEE IF THE RUBBER ON THE MOUNT IS BROKEN.....OR SPLIT.....

 

I just had the drive shaft in my 99 cc auto redone at a local shop, but it all started because I noticed a vibration between 60 and 75 mph. Turns out it was caused because the U joint caps in the rear were bad/had been bad for a while.

This seems to be a common issue with our older super duties - The straps that hold the U-joint to the yoke on the diff stretch, this allows the bearing cups on the u-joint to spin in the yoke. The cups are hardened steel and the yoke is softer cast steel so the tabs and seats for the cups wear.

So what had happened with mine was the tabs that center the U-joint in the yoke got worn and the cups were moving outward in the yoke. One day I grabbed the driveshaft at the diff to check the U-joint for ware and it moved side to side in the cups about .050 to a 1/16th of an inch. I went to a driveshaft shop near work one day and he explained this all to me and gave me the name of a shop near work that could replace the worn yoke in the diff along with new straps and bolts. They also said that this is a pretty common thing with the early super duties and that mine was not the first they had seen this on.

So it seams that placing the straps that would the u-joint to the diff yoke should be on all our service lists at around 60,000 to 70,000 mile of use on them.

Not saying this is your problem but it will cause the center carrier bearings to wear faster than it should along with over parts.

 

Fixed it.
It was the u joint right behind the mid support bearing.
I had crawled under there and checked around some, and everything seemed tight and in good shape. Then, last week, I crawled under to check some of the ideas from this thread, and the drive shaft happened to be rotated in such a way that I noticed a cracked cup on the u joint. I just got done swapping it out, and it works great now.

 

VERY GLAD YOU FOUND AND FIXED THE PROBLEM.....