im pretty sure its pinion angle.
I had these wheels/tires on my 10 bolt and it was a smooth as ride. So, they are out.
U-joints I just had john swap pauls from his driveshaft, onto my driveshaft when I did the swap. I also greased them. They are good. My driveshaft had no vibration with the 10 bolt, and I havent hit anything/dented or anything since the 12 bolt.
the only thing I did different other then the 12 bolt swap, was now I have pauls spohn torque arm thats a long tube chassis mount and the one I had on my 10 bolt for a while wasa bmr trans mount.
I have a magnetic angle gauge, its just a bitch to get under my lowered car and adjust it. Maybe I will cut up some boards to drive up onto to give me enough clearance.
I will figure it out.