had this prob on my old mx73 5m-ge powered croozer , where if the car was at a standstill, i wouldnt select reverse.
could be attributed to auto tranny leak?
not sure, just sold the car it before it totally died on me.
(for the time being)
if you want to get it into reverse, just put in Drive and let car roll forward, then while the car is creeping forward, put into reverse. somehow it grabs reverse!
disclaimer: no garantee it will work, but worth a try!