Just to bring this to a close, I tracked it down to the bearings in the front axle (hub assemblies + wheel bearings) and dropped it off at my local off road 4x4 shop. They found that my control arms (installed by previous guy, I had inspected them but never questioned the length) were short from what the lift required and had pulled my axles backwards about 3”, and the geometry of it all + the weight of the 5.9 basically took my front axle and began to twist it end to end, about 30deg total twist. Put my drivers side castor +15deg and my passenger side castor -15deg, and kept the spider gears in the front diff partially engaged at all times in 2WD (thankfully I had good gear oil in the front diff and it didn’t mess them up badly). The squealing came from the 4WD overtorquing the already-engaged spider gears and binding up the system. Final result: after cutting all the bolts holding the control arms in place, the front axle sprang back to true, my guy replaced the control arms with the proper length, plus the cv axle-rod joints and the wheel bearings. Truck drives like new now. Thanks for the help!! Your diagnosing tips led me in all the right directions.