Somebody once told me that when repairs exceed 25% of the car's value (assuming not a 1k junker), it is time to think long and hard about the repairs vs replacing the vehicle.

My thoughts
1) Be diligent on the cheap fixes / other causes (seems lot like a lot of people here have good suggestions)
2) is the x5 4.4 still the right car for you, if so figure out the cheapest / best way to repair the tranny vs getting another x5
3) if the x5 is not the right car for you, now is a good time to switch before sinking more money in the car

GL