2000 BMW 528 will not go into reverse

I believe that the car is a 2 wheel drive. In April 2009. The car would not go into reverse. Went to a BMW mechanic and a BMW dealer. When both the dealer and the independent mechanic hooked up the computer, they did not come up with any problem. I went to a transmission shop I was told that they would drop the pan and see if they could fix the problem. They said that there was nothing they could see and recommended replacing the transmission. Along with the transmission, the TCC solenoid was replaced. Subsequently, I have not been able to find reverse for about 4 times since April. I would keep driving the car foward and eventually found reverse. Today, I was not able to find reverse. Pushed the car into the parking stall. I have been told that the transmission is a self contained GM tranmission and once it is gone. It is gone. Please advise. Thank you.