2004 FX4 Crewcab 5.4
194xxx miles

I am getting a really bad vibration anything above 65-70 mph and gets worse going faster. Will happen in 3rd & 4th (overdrive), torque converter locked and/or unlocked. Not RPM dependant, only vehicle speed dependant. Seems to get a little worse after I let off the gas and let it coast. I can feel it in my seat and in the cab, not in the steering wheel. Its extremely loud and seems like its a "rotational" vibration if that makes sense. It does not happen when in park reving the engine, so I knows its in the driveline.

I had both differentials proffesionally regeared from a driveline company with 4.56 gears 6 months ago, from the stock 3.73s. I was running 33s at that time. I sold my 33s about 4 months ago with intentions to get 35s, but some other stuff came up and I have yet to get the 35s. So I have been running my stock rims with 32s. (yeah i know my gas mileage has been awful, running about 2300 rpm at 70mph). But I dont drive highway often.

When I first started noticing the vibration a couple months ago, I thought it was tires, so I rotated them several times. No luck.

Next I thought it wouldnt hurt to try putting in new u joints. So I replaced both staked-in u-joints with new Naepco ones, I have a 1 piece shaft. No difference. Thought maybe my driveshaft was bent or out of balance. So I went to a junkyard and bought a used one with only 60k miles on it, not bent, and had newer u joints in it as well. Installed it, no change.

Jacked up the rear end, put on jack stands, took rear wheels off, and put lugs back on to hold rotors on, started truck, put in drive and reved it up to 80+ mph while video taping the driveshaft, video showed driveshaft to be very straight and no play or wobble at all. But still had vibration in the cab.

Also replaced transfer case fluid with motorcraft XL12 transfer case fluid.

So that basically tells me that its definetely not the tires, and it is most likely not the driveshaft.... I am stumped, only thing I can think of it that its something bad in the rear end, or tansmission/transfer case....

Any thoughts or input what I could try next?

 

You covered everything I was going to mention. I guess something else easy and cheap, is to get the tires rebalanced.

 

Thanks for the response. Yeah I should probably get them checked just to be sure. I will be getting new 35s and new rims here within the next 2 weeks... Just racking my brain trying to think what it could be. I dont know much about transmissions or transfer cases. I do know quite a bit about differentials from research, but I still dont know where the vibration is coming from.

 

Brake caliper hanging up?

 

I agree with the stuck caliper. If that's not it, you may want to have your rotors checked for warpage.

 

No I really dont think it has anything to do with the brakes.. They are newer dimpled and sloted rotors and newer brake pads as well. Calipers are fine too...

Driving the truck this morning at 55mph in overdrive with converter locked to work it is a fairly smooth ride. Anything really above 60 I start getting the vibration and it is a constant rotational vibration and get way worse the faster I go....

Another thing I had me tioned above was that it seems more pronounced after I get up to speed and let off the gas pedal and coast... Sound like maybe a pinion gear/bearing issue?? I dont think it could be in the tranny or tcase because it shifts so firmly and flawlessly with no slipage or anything... So I am leaning towards the idea that its something in the rear end

Anybody else have any ideas?

 

It may be that the new gears are causing the drive shaft/transmission to spin to fast...beyond what it was ever balanced or designed for. I took a quick look at a gearing chart so do not quote me but if you use 4.56 gears on stock tires than the whole drive line (before the gears) has to spin like it was going 100 mph when in fact the wheels are only turning like it was going 80. If you put the bigger tires on it will slow down the drive line and maybe stop the vibration.

 

That totally makes sense to me and I have thought about that dozens of times before. Just like the engine is just running too fast for the speed of the vehicle, causing a harmonic vibration...?

Im just hesitant to drop $2500+ on new rims and tires to find out I need to rebuild the rear end or worse the tranny... I am 22 and just bought a house a year and a half ago so I am living on a pretty strict budget. But I suppose I will go ahead and get them. Plan on getting a set of 20x12 weel and a set of 35x12.50x20 cooper stt pros. Cant wait, and hopefully that will solve my issue. Fingers crossed and I will keep everyone updated. In the mean time if anyone else wants to chime in go right ahead! Thanks

 

I had a vibration issue with a C6 where the transmission output shaft bushing had spun.

 

Just did some more research on extension housing/tailshaft vibration and it sounds like sometimes there is a bushing that can go bad and cause a vibration? Anyone know anything about this?