[Clint]

I don't have any paperwork on it but in the engine compartment someone wrote "Transmission 128k" in there so I'm assuming it had work done on it then? That has all been checked at Nissan and our mechanic friend took a look at it and they both said it shouldn't be a problem. You don't think that that stuff would work?

Most likely the trans was serviced (fluid change) then and may possibly be the reason the truck was for sale. I don't know why the shifter position would change how it shifts.
Maybe it wouldn't be an expensive rebuild since it's still in decent shape.

Clint

 

[Clint]

Now that you said that I think I remember something about the clip holding the shift cable in place were dislodging and the cables were moving when they should stay put. I could be mistaken.
I just looked through all the service bulletins listed for that year truck.
You can look at them at All About Nissan and look up two items per day or subscribe for unlimited. You can access service manuals, bulletins, recalls etc. for all Nissans.

Clint