Friend's truck, 95 Automatic, 4 cyl, 2wd, 110k miles. Reverse went out yesterday. All other gears shift fine, but reverse does nothing. I was thinking maybe a solenoid in the tranny, but I'm a Ford guy and don't know much about Nissans except that I find them hard to work on. I wouldn't think it would need a tranny replacement with only 110k. If you could give me some ideas maybe I can help her (and get a date!!).

its probally a faulty clutch or a faulty band

First, if this vehicle's transmission has bands and further, if
they are the type which can be adjusted (not all are), have
them adjusted... this should help so long as each band's
lining hasn't worn too much. Otherwise, the bad news is
that you will be facing a costly repair if the drivetrain is no
longer under warranty. Also, every garage in town will tell
you they'll need to do a transmission teardown in order
to determine the exact cause of the problem. They'll be
right too. Why? Because it's just impossible to know for
sure what the cause is until the transmission is opened up.