1988 Nissan Maxima V6 Auto, rblt engine 18K mile

The problem may be as simple as a linkage adjustment. Have the shift linkage checked and properly adjusted first. A shop wont charge you any more than an hour labor to adjust it. If that doesn't correct the problem, you could have a variety of internal transmission problems; anything from a low reverse band worn out to a valve body malfunction. Don't take it to a dealer! It's standard practice for them to just condemn the transmission and put in a new one. Try taking it to aamco or lee myles. They'll actually find what part has failed inside and can repair it without putting in a whole new unit.

Sometimes its as simple as flushing the transmission fluid and putting new fluid in.

Give it a go.