Differential Unit problems of the 2006 GMC Yukon

2006 GMC Yukon xl - purchased in Jan. 2006, the vehicle was taken in for service 1 month after taking delivery for foul smell emitting from vehicle while moving. It was diagnosed as the rear differential producing the odor, they changed the fluid to the old synthetic type, and claimed it was the smell of the grape seed extract used in the organic new type fluid. This did not solve the problem, the next time the service center tried an oil which had a higher heat rating, this did not fix the smell problem. On the 4th visit to the service center and after opening a customer claims case with gm, they rebuilt the rear differential. This seemed to solve the problem for about the first 6-8 months, the smell returned and got significantly worse until I opened another case withgm customer claims. While working with them the differential went out on the vehicle leaving me to rent a car on my own until gm decided what they were going to do about it. I am currently in a bbb case with gm.