Check engine light is not on?

Professor
www.telstar-electronics.com

That sounds like a bad torque converter. I'd unplug the wiring harness
to the trans and try putting it in gear. It could be that it's being
told to always be engaged. Could be a bad solenoid in the trans too. The
last step would be to test the lockup solenoid before you pull the trans.

On some of the older Nissan's there was an unlocking lever next to or
under the main leaver. You will have to carefully look. If not, it is
possible that it is jammed and will need some maintenance.

1997 Nissan Pickup.  2.4L four cylinder, auto trans.

Idles fine, but put it in gear and it stalls out.

Any suggestions?