Sounds to me like you found your problem.
I would avoid driving in Auto4WD unless you are planning on seeing changing road conditions like in the winter. Definitely don’t drive in 4HI when on dry pavement. 4LO should only be used in severe situations. In fact, I told my teenage son that if he is in a situation where he thinks he needs 4LO, he better be calling me since it means he most likely f’d up and is in deep with need of help.
Oh, of course.
I know about the 4WD conditions and how it shouldn't be used unless needed or strongly anticipated (auto mode).
This 2013 Tahoe LT was driven by a teenager going to college (dad bought it) and she never really "challenged" this vehicle.
In fact, she appears to have spent quite a long time abusing the interior with food, drinks, coffee, makeup, and candy.
I also think that she had a chihuahua or something similar with her all the time in the vehicle as well.
She also spent quite a bit of time on the BEACH driving on SAND which I found everywhere, slowly sanding it's way through paint and rubber seals.
I deduced all this by the amount of short dog hair and sticky stains and crud I pulled out of the interior once I gutted the whole interior to clean and sound deaden the body.
I mention this as part of this thread because this thumping has gone away, and I'm betting that the transfer case, front differential, and front axles were never used much at all.
After exercising the 4WD hi and LO gear for about 30 minutes on various road surfaces, including gravel and grass, I'm hoping that i have "worked out" whatever was happening.
I'll keep an eye on this and come back here if / when it returns.
