At one point in my long and detailed history of repairs I too had to replace a rear transmission mount that was gone. I know that it was gone first because I got under in and around every part of my car before to the garage and second:
Because every time I put the car into reverse the loudest CLUNK sound came from under the car about midway from the front to the rear seats...I mean a CLUNK that made you fear the bill!!!!!
I hope that my 2 cents helps...
-G