Then I would have the rear end gears checked out. Some early posters to this thread had some success after having it rebuilt. Some claimed that they were assembled incorrectly with backlash and preload set wrong.
How did you achieve lower gear higher RPM? If you did this by putting the tranny in manual mode, then note that M5 gear disables AFM, so perhaps you are getting a vibration due to AFM? Also, if this is by accelerating, then this will change the torque on the gears, particularly the rear end, so there could be a gear preload issue (this has been speculated back a ways in the 800+ posts). Accelerating may also cause the axle to wrap up some and change the pinion angle, which is another possible cause that has been speculated before.
I have been following this thread for a long time, and from what I remember (you can always go back through all previous 828 pages :-), the problem has been reported pretty much on any and all configurations, all the way from WT up to LTZ to Sierra Denali's. Regular, double and crew cab, 2WD and 4WD, steel wheels and aluminum, low profile tires (20+") and standard profile (17-18"), different trims (high country, etc) No real rhyme or reason. Lots of different fix attempts, some things have helped for certain people, some not. What works for some doesn't work for others. Good luck!
You can put the trans in manual mode, gear 5, and it will not go into AFM. Your RPM's will be higher so your mileage will suffer, but if you just want to see if AFM is the culprit, this is a way without a programmer.
I believe, somewhere in the past 712 pages of posts, that someone tried this (at least the front shaft) and it did not fix his problem. This doesn't surprise me since both 2WD and 4WD trucks appear to have the problem. Now, 07SoftTail claims he fixed his by having his rear drive shaft trued and balanced, but others have also checked their driveshaft and it was fine. There was also (again a few pages back) talk that the rear axle flanges were badly machined and you could clearly see that they had a bulge on one side. If you go back through all the pages, you will find that these and many more things were already tried. Some helped for a few people but not others. It seems there is not necessarily one specific problem that you can pinpoint for all trucks.
This is torquing 101. You never torque each bolt/nut one at a time to the final torque value. You start with one, thread it on finger tight. Then go to the next and do the same thing. When all are then finger tight, you start torquing them down a little at a time. You do each bolt/nut a little at a time before going to the next one. You go from each bolt/nut to the next in a specific pattern. For wheels, you go across to the opposite bolt/nut to get even torquing. Make 3 or 4 passes around using this pattern, tightening more each pass. On the final pass you torque to the manufacturers specification. On some things, like cylinder heads, there is a specific pattern that the manufacturer specifies, you can usually find this online.
Which, of course, is ridiculous because there have been plenty of class action lawsuits for silly things that didn't kill anyone like extra fees being charged on cell phone bills or improper medical insurance billing. I figure it again is just not widespread enough for there to be any money in it. A lawyer won't take a case if there isn't enough money.
Another thing to do is to use an indelible marker to mark the sidewall on each tire where the valve stem is. In previous posts (somewhere back in the 629 pages), some posters had noted that the tire was slipping on the rim. If it does, then the RF balance is no good any longer. This way, if you start to get the vibes back, you can easily see if the tire has moved.
Except he noted that the drivers side has play and the passenger side does not, which makes no sense unless he is talking about the shafts to the front wheels, in which case they would be the CV shafts not the driveshaft, and they would unbolt as a single integrated unit. Note the driveshaft hanging down behind the diff in the right side of the picture.
I got the impression this was the FRONT differential, so he should not be dealing with U-Joints. Also, if it is the front, then if a loose bearing is the cause of his problem, it isn't the fix for everyone because it would not explain the 2WD trucks that have the vibration.
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