Hello Adam. I have the same truck, 15 Sierra Black Widow with 6" fabtech lift and experiencing similar issues. I bought it brand new. My lock up when turning seems to be a little worse than yours. I don't even make it to the steering locks and can barely back out of or pull into a parking spot. I get to about 10 or 12 degrees of turn and it is like I am applying the brakes, in forward and reverse. If I turn much farther, The truck will actually lock the outside wheel and stop the truck all together. Looked underneath and there are no signs of rubbing what so ever.
I am having some other issues as well. I believe my lock up is due to the fact that my truck has decided to engage 4WD Hi on its own. I've had the truck for 7000 miles and left the 4WD selector switch in 2WD since I owned it. I took it into the dealer for the turning issue and they told me the truck is in 4WD Hi and they can not get it to disengage. I have an appointment to take it in in about a week. I will post back here if the turning issue is resolved with the fix to the 4WD system.
My other concern is what damage has been done to the front transfer case since it has been driven in 4WD Hi for 7000 miles.
Secondly, it seems my torque converter is having issues. There are times when I put it into drive and the tranny does not engage until after I apply the accelerator. This results in a type of launch situation that will actually screech the tires. I also have experienced similar issues while driving, both accelerating and decelerating, from a stop and while underway.
Just this morning a new issue started. a couple seconds after starting and in idle during warm up, a loud moaning started. Sounded like it was coming from the tranny or transfer case. Increasing the idle while in park or neutral did not change the tone or loudness, but when I started driving, the loudness did increase. Eventually it went away and then became intermittent during my 45 minute drive to work. It seemed to only return when the truck was not under load. In other words, when going up a hill or coming down a hill you could not hear it, but while on flat road sections it came back. I will see if it is still there on the way home.