My name is XXXXX XXXXX I will answer your question. Without actually performing tests on your vehicle, all I can do is make a educated guess of what has happend to the trans. Needless to say it is going to be a part of the unit that is only used when in reverse since you stated that when in "D" it works fine. There is a reverse one way in this vehicle as well as a reverse clutch. Most likely one of them has failed.
In order to determine what has happened, First a tech should hook up a electronic transmisson tester to determine if the problem is being caused by internal electronics or not. This would also be after first testing the vehicle for any DTC's that may be stored. then a Line pressure test should be done to make sure that the line pressures in the transmission are correct when it is put in all of it's manual selections. The last thing to do to determine what system has failed is to take off the valve/solinoid body and perform a "air test" where a plate is installed and air is applied to the ports in the plate and the tech listens for the different clutchs and bands and pistons to be applied. If you dont hear one of them working when this is done, then that is the area that you need to go after that is causing the concern. Of course, at this point, removal of the transmission will be required.