Not sure if this is the correct section to post this but, Just finished getting the 4 in. Rancho lift installed along with changing the gearing to 456 for 35in toyo's.
A few miles down the road the rear end failed. Turns out that Yukon says the ring/pinion was defective somehow and replaced it NC and no other damage done. All was fine for a day and now there is another rattle/grinding noise from the front this time. It is not the same type of sound since the rearend noise was obvious and you could feel it. The front is a lighter sound that only happens when applying gas and not always. It is not as rough and there is no vibration felt. As soon as I take my foot off the gas it stops and when going down hill there is no problem even at 55mph. It also doesnt seem to happen in 4 wheel drive. The truck spends about a third of the time off road and in 4 wheel. All the suspension seems tight & sounds good.
Question is, has anyone had any issues like this with Yukon gearing or does anyone have any ideas what this may be if not the gearing or any suggestions on what to try. My truck is an 08 screw w/5.4. Other than this problem the truck is great.! Sorry for the long story but I'm gettn frustrated.

 

not a gear expert, but could it be the gears "breaking" in?

 

Thanks for the reply. I wish that was it. I installed new gearing in a 71 Bronco a few years ago and besides taking it easy the first few miles and not towing for a few more there was no sound or issues for front or rear through the whole break in. I'm also wondering if there could be something going on with the auto engaging 4x4 This is the first truck I've had that I dont gotta go outside & lock/unlock the hubs. But then there was no problem until the lift & gearing was changed.

 

id send mercedestech a pm. hes really knowledgeable and im sure he will have a few guesses as to what is up. or search the forums for post dealing with gears and see what you come up with

 

Maybe they didn't but in the friction modifier. These truck MUST have it in.

I would ask them if they used fully synthetic 90W-140 and also added the friction modifier that must be bought from FORD

 

Thanks for the info, hoping to take in to the installers today to see what they think. I'll ask about the fric modifier.

 

Well took the truck into the shop and on the way in there was no noise or vibration. The mechanic looked it over real good and could find nothing externally wrong so we went for a drive, about 15 miles in 2wd and 4wd, rough road, smooth road and nothing...So I am going to just keep driving it. He thought it may have been a rock sandwiched in the brake rotor. Could be but how could it only make the noise when under load?