transfer case fluid

[manfrmbklyn23]

I have a 04 4.4. since I have owned this car (7 months) it has been having tranny problems. mostly when cold the trani would slip and create a hard thud,almost as if being rear ended. bmw has reprogramed or upgraded the software the 1st time at the dealer. on the 2nd visit for the same thing they replaced the mechatronics(have no idea what that is). Now on its third visit (technically 4th visit, they denied any problems on the 1st visit) they said BMW had a bulletin out for the transfer case fluid. they replaced the fluid (SIB-27-01-07 vtg fluid is what my reciept says). the car is a CPO and still under warranty, they told me it wasnt covered but they would do it under good will since the car has been there so many times for the same thing.

Has anyone had this problem? the car only has 46000 miles. what exactly is the transfer case? thanks in advance.