Tuesday night I noticed some bucking during light acceleration in 4th gear. The whole truck was vibrating very quickly. It seemed to shake from the floor up. The steering wheel was not shaking but even the hood was shaking with the vibration. It seemed to be worse on rough roads and only occurred while accelerating very lightly around 40 miles per hour.

Today I was towing near 7,000 lbs worth of trailer loaded with Highland coos around the Ozarks and the shuddering was worse. I started while the trailer was empty, only weighing around 4,000 lbs. The shaking now occurs at higher speeds roughly 65-70, though it also happened once in the mid 50s. Never happened at lower speeds today but throttle input was greater due to the load.

The shudder has become far more aggressive with the whole truck shaking violently. As far as noise, the sound of the whole truck shaking is quite loud so I couldn't aurally pinpoint a source, just a loud roar. We pulled over at one point to check things out since the problem was so consistent and scary. 4 wheel drive is not engaged, the rear driveshaft seems normal with no slop in the u-joints when shaking by hand and watching it with lots of testing in a lot.

I'm suspicious of the transmission. I would expect that if the transmission is slipping that the temperature would increase. It did seem higher than usual, around 145 on the interstate though it never exceeded about 171 degrees even when pulling through muddy pastures, water holes, and steep hills off road. The transmission used to run under 135 even in the summer. Ambient temperature was about 63 degrees today so it was not exceptionally warm.

Fuel pressure is mostly in the upper 20s but is not varying enough to be the cause, I think. Tires look perfectly smooth while driving. Using a laser thermometer gun my rear differential only got up to about 180, as were my rotors. I have sticking caliper squeal from the rear but I feel no pull. I find it hard to imagine a sticking caliper that is not slowing the vehicle could cause such a harsh vibration.

The way the truck is bucking it feels like a loose throttle spring. I say this primarily because the shudder is usually triggered by rougher terrain such as a bridge, around 70 mph, under 25-35% light throttle acceleration. The bucking continues after the initial vibration until I either accelerate or decelerate.

I've tried downshifting into 3rd gear while maintaining speed and the vibration ceases. This again makes me suspicious of 4th gear. There are no changes in shifting behavior, timing, or feel. The only slippage indicated by my PCS TCU is when I shift between 3rd and 4th gears for just a second during the shift. The transmission oil is full and still has great color, no odor or debris. Brian of BTS told me not to worry about changing the filter or oil until it looked bad so I never had in the years I've been using this 4R100.

I'm also considering the problem could be the driveshaft going out of balance. Or perhaps my engine's timing or GSK has slipped. There is no engine smoke and vehicle speed and engine RPMs do not bounce with the truck, which surprises me since the vibration is so harsh it is impossible for me to keep my foot steady on the throttle.

I'm hoping someone may have had a similar experience and found the cure. Otherwise, does anyone have any other ideas or steps I should take toward the diagnosis? I am not sure if I should try to find a good diesel shop or drivetrain shop. I don't know of a good one of either in the area but money is extremely tight so it would be best if I can figure this out and repair myself. For the moment, I'm puzzled.

 

Today I used my laser gun on the u-joints and they were actually cooler than the ambient temperature of 48 degrees, being about 43 themselves. I also got temperature readings from all of my hubs, tires, and rotors. One rear rotor was about 118 degrees compared to the other side around 80. Probably the one with a sticking caliper. Tires were all between 68-80 something degrees on the tread surface. Hubs were ambient.

I had difficulty duplicating the vibration during over 100 miles of mostly interstate driving today. Sometimes I felt a light vibration around 57 mph, the worst all day was at 70 mph again but nowhere near as severe as yesterday with the trailer. Though the shaking was mild today I still noticed it occurring with the throttle position between 20 and 35% on my laptop. I tried recalibrating the line pressure in 4th gear so it was on as strong as possible and noticed no change other than firmer shifts. Transmission temperature during interstate driving today was 115-120 degrees at the hottest, running at 118 as an average during cruising.

I'm leaning away from a transmission problem for now. I figure I should try installing a new throttle cable since this one pops off the engine. I got it from a salvage yard off a V-10 Super Duty a few years ago. It has had paper towel stuffed in the socket to stay tight but still pops off occasionally. I don't think that looseness is the problem because it hasn't popped off in months and if it were that loose with the intense vibration, it should have come off the throttle mount by now in the last couple days. I'm more suspect of the throttle return spring built into the throttle cable. When the vehicle is stopped I have no way to replicate the speed and vibration but since the cable should be replaced anyway it seems a logical first step.

I'm also going to get my tires rotated and balanced again. The Toyo Open Country M/T 37x13.5" tires are down to about 9/32nds with around 20,000 miles. I had tried an internal bead type balancer several months ago that shook the truck very similarly right around 61 mph and sometimes slower. Went back to sticky strips and no more vibration. I have a few thousand miles on this balance and rotation. I bought the tires from Les Schwab with a free lifetime balance and rotation on their Hunter GSP 9700 road force balancer. They advised that these particular tires lasted longer and rode smoother with a static balance and rotation every 1,000 miles. Unfortunately they have no stores in Missouri so I'm going a few thousand miles per balance and rotation. But I think it's got to be drivetrain related.

 

Figure anything out?

 

The double cardan joint was completely worn out. Not just the u-joints. The entire thing was worn beyond rebuild, it required replacement. The shop had never had such trouble taking one apart. They said the pieces usually fall right out but with these they had to beat them apart.

Most of the vibration was taken care of, definitely the extremely harsh shaking. The truck still vibrates more than it usually did, particularly over 60 mph. The tires have very little tread left and I hope to eventually find time to search for more possible causes.

But for certain, the double cardan u-joint parts were completely toast. The shop suggested I delete the factory vibration dampener on the driveshaft. They expect I might double the life of my double cardan joint by doing so due to it's tremendous weight. They said the vibration dampener should only affect noise, not vibration. I haven't had time to try it. It will take some serious metal cutting to get through that dampener, and I guess most of the rubber will remain.

 

I am experiencing the same problem. I replaced the tires with little impact. Took it to the shop today to get it diagnosed. If you don't mind me asking, how much did it costs you to replace the double carden joint?

 

I spent around $350 to have the entire rear driveshaft refreshed. New U-Joints front and rear, balance, and new double cardan. You might ask the shop about removing the vibration dampener, I have yet to hack at mine.

My Dynatrac Free Spin front left wheel bearing has to be retightened about every 1,000 miles so I'm way overdue, but yours are probably stock. The vibration I'm still experiencing is not in the seat, but in the front floorboard through my legs, and slightly through the steering wheel even with the hydraulic assist ram. The dash makes noises from the vibration while driving. Not very noticeable to most passengers but to me it is very significant.