Ok start this with mods list. 2009 Sierra 4dr 4x4. 7.5 RC with 1.5 Body lift on 37 13.50 18s. Fabtech uppers, new inner/outer tie rods. Wheel bearing check out no issues, replace 2 years ago with oem just due to time and miles.
Front shocks are 3 years old Bilstien 5100s. Rears are Brand new RCX 2.0.
Truck has 165k. Owned it since 50k Nothing has ever failed and it is the stock gearing, trans, rear end etc.

First major issue, steering issue happened twice within a week with no obvious signs of wtf it could be.
While slow turning like backing out of a parking space almost full lock then as i start to drive and straighten out the steering wheel does not return to center, it stays around 9-10 clocked position and truck feels crazy weird, Traction control and Stability control throws dash codes during this time. I pull over or stop, turn steering wheel full lock left then right and it returns to center, turn off truck and restart all dash lights clear out. Assuming Steering position sensor tripped? Steering rack failing? Only thing i thought of is while doing body lift install 3 years ago I did not adjust the steering shaft or loosen it during install but damn that was 3 years ago.

Second Issue:
Vibration, I have had a vibration at highway speeds of 65+ for so long i cant remember. Pre lift kit it was there not as bad but the rear right tire ALWAYS wore out and got chopped. Now since ive gone from 35s to 37s even after new rear shocks it is rough . especially in the back seat. Shakes like a MFer. It is intermittent depending on speed and whether in a curve is the damnest thing. I freakin cant figure it out. Stock rear end and gearing.

Wheels and tires ruled out of scenario as i have multiple sets

Sorry for long post. I just keep throwing parts at it and im tired and got other projects that need attention and money as well.