Discovered something interesting yeasterday, For people that have a Crerw Cab try going down the road at 60MPH and only roll down your back passenger window, it gives the worst buffeting sound ever and starts to shake the truck side to side just like when the truck starts shaking with the windows up, i really think that in a lot of cases the people that have the shake on and off that its something to do with the way air is flowing over and around the truck, any change in the flow makes it start shaking. This is just what ive noticed with my truck. I can also watch my roll up cover in my rear view mirror and see the flapping of the cover goes with the shaking of the truck when it is doing it.
I agree with you, mine is the same way, some days the same road in the morning it will shake a little but not on the way home and some days it wont in the morning and it will on the way home, sometimes it starts at 72-73mph and sometime 77mph, i can watch my roll up cover on my bed while its shaking and it is fluttering in the same manner that the shake is. Not saying its the problem for all, but in a lot of peoples case sure it is, because when everything is good that rotates it has to be air flow/air density.
As long as you have had it to the dealership before for the Vibration before the warranty ran out it should be covered after warranty runs out because it is an on going issue that has never been fixed.
Im not saying its your leveling kit, im saying its probably your rear drive shaft angle when under hard acceleration at high speeds over 75mph like mine was. If you check the angle while sitting on level ground it could be ok, but when high torque is applied to the rear end like while driving at high speeds and then flooring the gas peddle the angle can be way out of spec, i think the reason for this is the spring are to soft for the first couple inches to give a better ride which then makes the differential twist up and down to much, that is why if you try the leaf spring clamps behind the block it will stiffen up the spring pack so it dont twist as much.
You will have to take it to the dealer and have them ride with you to hear the noise, they will not take it to that speed or drive it aggressive, and if it is the pinion twisting up under hard accel at high speeds then they need to put shims in, the rear wedge shim they put on mine is additional to the block that is under the leaf springs, and they put spacers between the transfer case and the mount but on yours they would put them under the tranny. You could also go to the part store and get some leaf spring clamps and put them 8'' behind the rear leaf spring block, that will help stiffen up the spring pack and stop the pinion from twisting up as far, it wont hurt the ride of the truck either. tThe clamps are cheap also You will have to take it to the dealer and have them ride with you to hear the noise, they will not take it to that speed or drive it aggressive, and if it is the pinion twisting up under hard accel at high speeds then they need to put shims in, the rear wedge shim they put on mine is additional to the block that is under the leaf springs, and they put spacers between the transfer case and the mount but on yours they would put them under the tranny. You could also go to the part store and get some leaf spring clamps and put them 8'' behind the rear leaf spring block, that will help stiffen up the spring pack and stop the pinion from twisting up as far, it wont hurt the ride of the truck either. tThe clamps are cheap also
The shims they put in go under the the transfer case or in you case under the tranny mount and the ones they put in the rear are a wedge block that pointed the pinion downward more so when a lot of torque is being applied the pinion doesnt twist up to far
I had the same issue and it ended up being my drive line angle was off, I also have a 2'' leveling kit. They put shims in the rear with the larger end facing the front of the truck and shimmed the transfer case up. What was happening was under acceleration while moving the truck that fast the engine was applying a lot of torque to the rear end which twists the rear pinion upward and makes the drive shaft angle off. Mine would only do it when i was traveling 75mph or more and accelerated hard. The joints need to cancel one another out with torque applied to them. I also have the 6.2 engine with 373 gears, so that also applies more torque to the rear end.
Your vibration is probably coming from exhaust harmonics, mine does the same thing, i can hear the exhaust tone and feel the harmonics at the same time and only when giving a little more gas to speed up on the highway or when the engine starts to lug going up a hill.
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