1993 Nissan Maxima Gear problems

There is nothing else you can check. A shifting gear is worn-out or broken in your automatic transmission. You need to take your car to a transmission shop and get the transmission repaired (which will cost almost $2000) or put in a good used transmission (purchsed from a salvage yard) or you can just junk the car. If it were me I would just junk the car (could come down with an engine problem soon after the transmission is fixed) and buy another one.