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Discussion Starter #1
Even after new tires, wheels, getting them balanced, getting the driveshaft balanced with a new joints and carrier baring installed.....I still have a slight vibration I can feel in the seat at around 65-70mph. Under that it's smooth as can be....over that it's smooth as can be. Any ideas as to what could be causing it? Ordinarily I wouldn't care....but it's right at the speed that I cruise down the highway at...so it's annoying. It's a very high frequency vibration....not really like an out of balance tire. More like a driveshaft type vibration....but I've already had that balanced with all new u-joints.

Are these trucks just prone to some odd resonance type vibrations at difference speeds?
 

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Just about every Excursion and F-250 I've seen has a large harmonic weight attached to the transmission crossmember. Does yours have that? Other that than, I'd be looking at motor and tranny mount condition after all you went through to make sure things are balanced.

Another idea....find out the exact RPM when this vibration happens. Now, put it in a different gear and cruise at that RPM. Does it vibrate? If not, keep going up to 65-70 in that lower gear (assuming you're not using 1st lol). Does it vibrate at that speed and higher RPM? If it's RPM related, you may want to look at injectors causing a slight misfire or imbalance. Maybe a contribution test would be in order?
 

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Discussion Starter #3
Just about every Excursion and F-250 I've seen has a large harmonic weight attached to the transmission crossmember. Does yours have that? Other that than, I'd be looking at motor and tranny mount condition after all you went through to make sure things are balanced.

Another idea....find out the exact RPM when this vibration happens. Now, put it in a different gear and cruise at that RPM. Does it vibrate? If not, keep going up to 65-70 in that lower gear (assuming you're not using 1st lol). Does it vibrate at that speed and higher RPM? If it's RPM related, you may want to look at injectors causing a slight misfire or imbalance. Maybe a contribution test would be in order?
I don't have a weight on my driveshaft.....and the shop the put the new joints in it and balanced it never mentioned anything about having a weight like that.

As far as what RPM is vibrates....it's speed dependent, not RPM/gear dependent. I can even drop it in neutral when running 75mph...and as soon as the truck slows to 65-70mph, you can feel the vibration...when it slows down under 65mph it goes away.

I'd say it definitely a driveshaft issue. Could slop in the slip yoke splines cause that kind of vibration?
 

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Sorry, I wasn't meaning a weight on the driveshaft, but hanging from the transmission/transfer case crossmember. Attaching a pic for reference. I know without those, sometimes a high pitch hum can come through the frame. I'll attach a pic.

If it's a high pitch vibration, it could be the splines. Has the slip yoke been well lubed/packed lately? That can cause some vibration. If you're sure it's not tires, and the shaft has been balanced, I would consider the yoke packing as one cause. Check for free play on the output shaft of the tranny/transfer case and on the pinion of the axle. If those bearings have up/down/side play it can cause vibration like that. Otherwise, maybe make sure your transmission mount is good, because if it has allowed your tranny to slip sideways too much, you could get a mis-alignment.
 

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