Yukon gmc 99 Denali vibration when coasting

[TDTM]

So I am wondering how I tell which transmission I have on my 1999 gmc Yukon Denali. I have a bad vibration on coast I have changed both u-joints in the rear shaft already and it happens in 2wd. My 4wd is electronic shift.  Any ideas on issues and which transmission I’d have or a way to find out? I’ve posted my RPO codes as well 

[richard wysong]

have you checked the oil in the transfer case? The chain in the TC wears into its oil pump so the fluid will have a metalic look to it. Also the rear differentials had problems on those, Did you make sure the ujoints were centered when you changed them and there was no change?

[biscaynekid]

By ‘coast’ are you in Neutral?

If not, build up speed then coast in Neutral.

 

If vibration is there, I would check wheels/tires and possibly bent drive shaft.

[TDTM]

You let off the throttle is what I mean by coast in drive still. And I did center the u-joints. The vibration feels like it is directly under your feet. And it modulates at 100 kmph or higher 

 

I did also change the output seal on the transfer case to the rear shaft because it was leaking and topped up the transfer case about 700ml of fluid I used the transfer case fluid from gm as well. And the vibration was under the drivers side floor panel before all of those changes. And still there now 

Edited June 2, 2025 by TDTM

[biscaynekid]

I would still check to see if vibration stops while coasting in Neutral. This releases tension on drive train. If it goes away, I would check rubber mounts for tranny, drive shaft , ect.  
If vibration continues in Neutral , again, I would check tires/wheels, drive shaft.

If you have a stabilizer shock absorber going from body to frame, check that too.

[richard wysong]

I would check to make sure the front axle is disengaged, the actuators get corrosion around the threads that compresses the housing and the dildo gets stuck, 

[davester]

charm.li will have the meanings for most of the rpo codes shown (some are internal gm codes), so decode them all yourself.

 

easy way to check that the 4wd system isn't engaged is to reach under the driver's side of the truck, and try to manually spin the front driveshaft.  If you can spin it, it's disengaged.