Normally for it to go into "Limp home mode" it is overheated (Code 33) or truck has a low oil pressure issue.

Nissan also has a service bulletin out about the transmission sensor on left hand side of the transmission getting wet and will do the same thing, just remove sensor,clean it good, reinstall and then silicone it to seal it.

Not real sure but check under the dash on right hand side, there may be a OBDll connector in the harness you can hook a good automotive handheld code reader up to, It'll give you a P code, but we can figure it out from there.

Don't your model, but do know Nissans quite well, is there any codes showing on the dash in the hour meter? I'll do what I can to help you out.