general problems of the 2012 BMW X6

My car pulls hard and shudders when turning left or right or reversing at a very slow speed. Dangerous to drive shops have told me but they are having a hard time finding what issue is. My brother said it feels like the transfer case or all wheel drive not working. Shakes hard at every turn like wheel is dragging. My turbo's have been leaking since the day I got car and BMW never advised me after I have repeatedly told them my car isnt driving at max potential. Im always told a lot of white steam or smoke coming out of exhaust is normal. Have had my car at BMW service more than ive been able to drive it. Replaced car battery myself because car kept saying low power mode and BMW service charged me $800 saying it was negative battery cable. . It wasnt the issue. Car went into limp mode a week later. I couldnt drive it for 2 weeks then finally had it towed to a regular shop which said it was the spark plugs! now car transfer case has been bad for past 6 months.