Help me diagnose a problem

[Omcentarfer]

2007 (NBS) Sierra Denali AWD 1500.  160k miles.  In the 100k miles I’ve had the truck, I’ve done a sensor or two, ball joints, cv axles and wheel bearings.  Truck has original (transmission, differentials, drivetrain, etc.) 

 

Around 3 weeks ago I was pulling a 12ft utility trailer, (maybe 1500lbs loaded) and I noticed a weird shake/shimmy starting around 40mph while the truck was under load and with throttle.  Notice it most when truck shifts up, and maintains a lower rpm.  Fast forward to now and it seems to have gotten worse, I am occasionally hearing a little noise that starts around 15, ends around 30, but still the shake @ 40 ish.  The shake gets worse and worse as speed goes up.. BUT ONLY under acceleration.  If I take my foot off the gas, it stops.  
 

I’ve read it could be ujoints, TC, transmission, front end issues (since it’s AWD), rear end issues, sort of all over the map.  
 

I will say this transmission has the clunkiest and clankiest transmission I’ve ever had, but it seems to be hanging on by a thread and it’s been that way the whole time I’ve owned the truck.  This issue doesn’t seem to impact shifting at all, if anything the shake might stop if for only during a moment during the shift itself.  
 

Would appreciate any ideas you have, things to look for.   Going to be hard to put a 4k transmission in a 7k truck, so just trying to see what I can do or should do.  

[richard wysong]

Sounds like a bad u-joint

[CamGTP]

I agree, sounds u-joint related.

[Omcentarfer]

Not u joint related.   Swapped both u joints on main drive shaft and swapped front AWD driveshaft and U joint and the vibration/shimmy hasn’t changed.   I will say my old ujoints were shot, doesn’t really feel like a waste.  

Edited September 16, 2024 by Omcentarfer

[Omcentarfer]

@CamGTP @richard wysong

[asilverblazer]

When it is vibrating, lightly rest your foot on the brake pedal - just enough to turn the brake lights on - while maintaining the level of 'acceleration' you were. 

 

This will cause the torque converter to 'unlock' or cease its attempts to do so.

 

Could also be a miss fire but I would expect a check engine light.

 

Maybe driveshaft is out of balance.

[Omcentarfer]

Welp, did both front wheel bearings just for the hell of it… no such luck.  I just feel like it has to be transmission, transfer case or front diff related.  
 

Once the “shutter” starts braking doesn’t stop it, but letting off throttle and it immediately stops. 

[asilverblazer]

14 hours ago, Omcentarfer said:

Once the “shutter” starts braking doesn’t stop it, but letting off throttle and it immediately stops. 

Can you explain this another way? Braking doesn't stop the shutter but letting off the throttle does - are you letting off the throttle when braking? When it is shuttering, do not let off the throttle (to keep it shuttering) and apply the brakes lightly. 

 

Try manually selecting a lower gear, neutral and using tow haul mode. Report any changes.

 

I would not consider the front differential.

 

Can the transfer case be locked in 4 high to bypass any clutch packs of the AWD? I missed the Denali AWD part and don't remember well enough its functions.

 

Try wiggling the output yoke of the transfer case and the input yoke on the rear differential. Confirm there is no carrier bearing and that if there is one it has no play.

 

Check the rear mounts of the transmission/transfer case. 

 

Does it occur at any speed? Any gear? Try loading the bed to squat the rear of the truck to significantly alter the driveshaft angle.

Edited October 4, 2024 by asilverblazer
Italics area above