If your 93 Nissan Maxima will not go into reverse at all, your transmission is either getting ready to go out or you are low on transmission fluid. You should check your transmission fluid first before messing with the actual transmission.
The reason a 1992 Nissan Maxima is downshifting so much is the transmission is slipping. This may be due to low transmission fluid levels, or old and worn out transmission fluid. The transmission itself may need to be rebuilt.