99 Yukon sq body, 249k, no codes, 4x4 has autolockers w/electric shift, auto trans.

Was out playing last night during 1st-rain conditions (slicker for sliding) and was very much in 2wd. Gettin' squirrely. Parked it around 10pm, rain kept on, went to drive it next morn at 10 & the front end drivetrain felt engaged, and there was a temporary grinding type of noise as I steered the truck left from the driveway and as she flexed down the rolling curb. It was not the brakes that made the sound, I recognized them doing their own grinding that you get when it rains and they get that layer of oxidation. So I shifted to neutral on column, hit my 4 low, it shifted and throttle response was appropriate, then did my usual flip to 2wd from there since it has never liked shifting from any other location back into 2wd. It did as it was supposed to, made the klunk sound and felt the little jolt but the wheels up front were still grabbing when I shifted and started driving. I stopped and repeated procedure, twice, three, and 4 times. Would not disengage the front drivetrain.

Nobody else had access to the vehicle to have gone out and abused it during the night.

A few days prior I think, perhaps only the day previous, my Yukondoit had begun it's day much the same but without the 4x4 aspect being present, or I simply did not notice it.

Crawled under it and found that the differential does have oil in it, everything looked as it ought to with the exception of oil and power steering fluid leaks that I didn't know it had because it doesn't drip. And I see that it has been a while since I applied any stopleak to her rubber components, AZ heat has everything looking cracked now.

I remember I used to have a 4wd problem before with the Yukondoit where there was this thunking coming from under the drivers side floorboard. I think I remember it occurred at low speeds and especially so in 4L. It was present in 4H but not as prevalent. And would all but disappear in 2wd; you had to hit a bump just right while turning left, such as pulling out of a driveway and getting the sidewalk-to-pavement bounce. That's when it would come out with a vengeance and shake a lot more than in 4wd, and I would have to slow down out of it or speed up and straighten out to alleviate. I have babied her for so long now I don't know that she will still act like that. Probably will. And she has kind of a lean-to-the-left stance like her torsion bars are unevenly set or theres a fat guy in it.

Hope you have a solution for me, good sir. Cause I have owned this Yukon for longer than any other vehicle in my life, and I have had some 48 cars now (by the time I became a level 34).